If I am on the ethereal plane and I can see the material plane, can I cast misty step to teleport back?
No, you cannot Misty Step back into the material plane.
The Dungeon Master's Guide explains (pg. 48):
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.
While on the Ethereal Plane you cannot see the Material Plane, rather, as explained in the DMG, you are seeing the points in the Ethereal Plane which correspond to points in the Material Plane. You can see everything going on in the Material Plane, but you are seeing it as it is projected into the Ethereal Plane.
Therefore, when Misty Step says (PHB, pg. 260):
you teleport up to 30 feet to an unoccupied space that you can see,
you can still only see within the Ethereal Plane.
Now, the DMG also has this statement about the Border Ethereal (pg. 48):
From the Border Ethereal, a traveler can see into whatever plane it overlaps, but that plane appears muted and indistinct, its colors blurring into each other and its edges turning fuzzy.
Is this enough to allow us to Misty Step to a point we can see on the Material Plane? Again, no.
This is just the explanation of how a creature in the Border Ethereal experiences the plane-to-plane correspondence mentioned previously - "every location on those planes has a corresponding location on the Ethereal Plane". You are still seeing points in the Ethereal Plane, not the Material Plane.
It is up to the GM how far apart planes of existence are
The misty step spell allows you to teleport to somewhere you can see; thus, if you are somehow able to see a plane of existence that you are not on, it would appear that you could teleport there. However, the misty step spell states (emphasis mine):
Briefly surrounded by silvery mist, you teleport up to 30 feet to an unoccupied space that you can see.
Whether an entire other plane of existence is 30 feet away or less is something that the GM is going to have to determine.
Here are some other questions about distances between planes:
- What is the movement distance to the Ethereal Plane?
- Can a portable hole be used to get around the 10' movement restriction on Glyph of Warding?
- Can I create a superbomb with Glyph of Warding and a book?
To quote the highest scoring answer to that last question:
It has been proposed that you might be able to get around this by using a portable hole or some other extraplanar storage see (this Q&A). This might work, except a portable hole is 10 feet deep so removing an item from it would still trigger it.
Also, a DM could argue that the extradimensional space of the hole is an infinite distance away from the current plane. Thus, taking it out or putting it in the hole would break the glyph regardless. So this comes down to a DM decision to make that call.
So it will be up to the GM to determine how far apart two planes really are. For me, allowing this to work gives a fun and interesting use for the spell so I at least would allow it to work (assuming they can actually see this other plane of course).
Something to consider when making a ruling is monsters which have truesight and a teleport, since they could effectively leave any combat or be particularly excellent at ambushing. Another thing to consider is spells like dimension door which have a distance limit but do not require sight; allowing them to go between planes would be a rather significant change to those spells.
For what it's worth, lead Rules Designer Jeremy Crawford has said the following in a tweet:
If two people are on different planes of existence, they are infinitely far away from each other. For example, if I'm on the Material Plane and you're on the Ethereal Plane, we're not within 30 feet of each other.
Of course, this is just what he happened to think at the time he posted that tweet, who knows if it has changed or even why he thought that ruling made sense in the first place.
Misty Step let’s you travel a certain distance in space. While the Ethereal Plane has locations that correspond to locations in the Material Plane, they’re not physically connected and you still need to step across plane boundaries to get from one plane to the other. As this Q&A spells out, there are a number of ways to enter or exit the ethereal plane, but Misty Step isn’t one of them.