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From what I understand in the references in the MM and PHB to Truesight, it seems that a creature with this sense can see into the Ethereal Plane and see invisible things/creatures. However, does this allow the creature to see through solid rock, e.g. if a PC was out of typical line of sight, with 100% cover, hiding behind a large tree or a stone pillar?

There was some confusion about this in a recent campaign. The way I would interpret it is that having Truesight does not allow a creature to see through objects, e.g. like x-ray vision. For me, seeing the "invisible" does not mean the same as seeing the "non visible".

I would appreciate any RAW answers using 5e literature and/or experience on applying a house-rule regarding this matter.

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No, you can't see through solid objects just because of Truesight.

Thomas Markov's answer covers the ability's phrasing and the Border Ethereal topic sufficiently.

I want to add onto that, however; Xanathar's Guide to Everything features an eldritch invocation for warlocks called "Ghostly Gaze" (p. 56/57):

Ghostly Gaze
Prerequisite: 7th level
As an action, you gain the ability to see through solid objects to a range of 30 feet. Within that range, you have darkvision if you don’t already have it. This special sight lasts for 1 minute or until your concentration ends (as if you were concentrating on a spell). During that time, you perceive objects as ghostly, transparent images. Once you use this invocation, you can’t use it again until you finish a short or long rest.

This invocation elaborately describes gaining the ability to see through solid objects and how you perceive them (as it would be very inconvenient if you couldn't see solid objects at all while using the invocation).

If Truesight were intended to grant the same benefit, we would expect it to have a similarly accurate phrasing. Remember: in 5e, rules only do what they say.

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    \$\begingroup\$ Note that echolocation cannot account for things behind corners either (in real life). It would require sound to bounce off another surface, travel behind the corner, bounce off you, then back to the first bounce location, then back into the creature's ear. If the creature can even perceive it, it's going to appear as if you are located at the first bounce location. This is mechanically equivalent to how you'd use a mirror to make it look like you were somewhere else. The creature would know you're somewhere nearby, but wouldn't know your exact location. \$\endgroup\$
    – Flater
    Oct 12, 2020 at 9:30
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    \$\begingroup\$ @Flater that's why I said it's not explicitly stated. I'm not an echolocation expert, so I wasn't sure if it would be possible with that sense. Blindsight, however, doesn't only include echolocation. A dragon's "heightened senses", for instance, may just be superior hearing or smell, or simply magical senses, which allow the dragon to look behind corners. Point being, it's unclear and depends on the DM. \$\endgroup\$
    – PixelMaster
    Oct 12, 2020 at 10:02
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    \$\begingroup\$ I know, I'm just mentioning it because your "it doesn't mention it but I interpret it" argument relies on real world common sense, no? I agree that "heightened senses" is vague, but I'd still argue that any non-magical sense is going to be directional and thus susceptible to the "mirror location confusion" (for lack of a better name). So unless the creature has magical affinity, I'd argue that it can't locate things hiding behind other things. \$\endgroup\$
    – Flater
    Oct 12, 2020 at 10:38
  • \$\begingroup\$ I recommend deleting the bit about blindsight -- it's dubious that blindsight allows one to see around corners, and IMO it detracts from an otherwise great answer. \$\endgroup\$
    – mdrichey
    Aug 10 at 16:40
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Truesight is not X-Ray vision, and invisibility is different from total cover.

Truesight says:

A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.

Nothing here says we can see through objects, and it is very plain to see that “invisible” is referring to creatures who have been made invisible by some sort of effect granting invisibility, not creatures with total cover. This is the important distinction: invisibility and total cover are distinct conditions. So the only question remains “what is it like to see into the ethereal plane?

I won’t reproduce the whole section on the Border Ethereal from the DMG, but I will comment that there does not seem to be anything that would imply the possibility of being able to see through solid objects, except for possibly this phrase:

solid objects on the overlapped plane don’t hamper the movement of a creature in the Border Ethereal.

While actually on the Border Ethereal, you can freely pass through solid objects, but it doesn’t say we can see through them while we’re there, and with Truesight, we aren’t even there, we are just peering in.

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No, you can not see through solid objects with Truesight.

The description of Truesight says

A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.

Nowhere does it say that the spell allows one to see through solid objects, nor that it makes solid objects transparent or translucent. In this context, 'invisible' is not a synonym for full cover.

The description of the Ethereal Plane (DMG, page 48) says (emphasis mine):

The Ethereal Plane is a misty, fog-bound dimension. Its "shores," called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane.

The emphasized part states that any location on the indicated planes has its counterpart on the Ethereal one: this means that walls, doors and the objects of a room in the Material Plane also have their respective counterparts on the Ethereal Plane.

Consider the following situation (see figure below): creature A (on the Ethereal plane) is spying on creature B, which is inside a locked room with thick walls on the Material Plane. A spies B from outside the room, seeing through the Ethereal Plane. B decides to cast True Seeing because it suspects that there are hidden traps in the room. A becomes potentially visible to B as if they were on the same plane, but since it is outside the room (i.e. it has full cover due to the walls) B can not see it, since Truesight does not allow B to see through the walls on the Material Plane. Once A enters the room in the Ethereal Plane, however, B can clearly see it (if True Seeing is still active).

B is in the Ethereal Plane and it is spying A, which is inside the room with thick walls.

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  • \$\begingroup\$ We might distinguish between locations and objects. Just because every location on the Material also exists on the Ethereal, doesn't necessarily mean that every door does, for example. Based just on the quotes presented, couldn't the Border Ethereal be a featureless plane that matches only the terrain of the Material, not its structures and objects? Are there other quotes that support your assertion (for example, the last one in Thomas Markov's answer)? Does it matter for your answer whether the walls and doors exist on the Ethereal? \$\endgroup\$
    – Kirt
    Aug 11 at 17:06
  • \$\begingroup\$ I never thought there are objects reflecting the material ones on the ethereal at all, where do you get this from? It only says there is a 1:1 mapping for locations. To me, that does not mean you can see back into the material behind a material object through the ethereal. You just see into the foggy mists of the ethereal. You would see an ethereal critter there even if it was in a spot that has a real world equivalent behind a wall in the real world, but you could not see that real world spot. \$\endgroup\$
    – Nobody the Hobgoblin
    Aug 11 at 17:55
  • \$\begingroup\$ @Kirt A "location" could be also an house, a building (see here): then walls and doors have the respective counterparts in the BE. The quote provided by Thomas in his answer indeed provides evidence that objects appear in the Bordered Ethereal, and one can pass through them. Moreover, if objects in the Prime Material are made by material coming from the terrain (ore, wood, stone, gold..) why the objects should not have their counterpart in the Bordered Ethereal? \$\endgroup\$
    – Eddymage
    Aug 12 at 7:19
  • \$\begingroup\$ @NobodytheHobgoblin See the above comment regarding the observation made by Kirt. \$\endgroup\$
    – Eddymage
    Aug 12 at 7:20
  • \$\begingroup\$ @Eddymage I don't see how that quote provides evidence for object on the ethereal, rather the opposite: It refers to objects on the material, which you can pass through while on the ethereal, which suggests these objects are not also on the ethereal. Your other line of reasoning is more useful, I think, because it gets complicated what you can stand on etc, if a house does not have a structural counterpart there. But maybe that is more of the kind of issue you have also when you consider if a large ship is moving or not for combat on it and fixed location spell effects. \$\endgroup\$
    – Nobody the Hobgoblin
    Aug 12 at 8:49
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Not in general; Taken strictly, RAW has possible exceptions

Truesight says...

A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them,...

Without specific language otherwise, we must interpret "seeing" in normal and magical darkness to mean regular vision, as solid objects are not producing either normal or magical darkness... but the rest of the portion quoted can be interpreted differently.

There is an edge case for seeing invisible creatures and objects that are behind solid objects. A strict reading of the statement that "A monster with truesight can, out to a specific range, see invisible creatures and objects" is that they can see all such creatures and objects within the range. Note the language in the comparable See Invisibility spell:

For the duration, you see invisible creatures and objects as if they were visible, and you can see into the Ethereal Plane. Ethereal creatures and objects appear ghostly and translucent.

In the case of that spell, it specifies that you see them as though they were visible. This indicates that it follows the normal rules of visibility, simply ignoring the "invisible" status. Truesight does not use such language, and does not establish what may block the ability to see invisible creatures, only the range of vision.

An even stronger argument can be made for "automatically detect visual illusions", given that "detecting" doesn't automatically imply a line of sight, the way that "seeing" does...

RAI *

According to Jeremy Crawford (discussing whether truesight allows one to see a hidden rogue),

Truesight pierces invisibility, illusions, darkness, and false forms. It doesn't pierce total cover or make you hear better. #DnD

He makes clear, here, that Truesight does not pierce total cover. Therefore, the intended interpretation is that Truesight doesn't allow one to see through real, physical objects that one wouldn't otherwise be able to see through, for any of its effects.

* provided as RAW doesn't sufficiently clarify the edge cases

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