Many creatures have resistance to weapons that aren't "magic weapons". A Magic Staff can be used as a quarterstaff, but (usually) doesn't have a bonus to hit (a "plus"). Is it considered a Magic Weapon for the purpose of damage resistance?
Generally 'no', but many staves' descriptions specify 'yes'.
Three illustrative examples from the DMG:
Staff of Power: "This staff can be wielded as a magic quarterstaff that grants a +2 bonus...." (p.202) Typical +2 weapon, magical damage.
Staff of Charming: "The staff can also be used as a magic quarterstaff." (p.201) Clearly a magical weapon, but no pluses.
Staff of Fire: (p. 201) No mention of melee use. So the general rule on staves, from p.140 applies: "Unless a staff's description says otherwise, a staff can be used as a quarterstaff." So it can be used as a quarterstaff, but not as one that does magical damage (by omission).
(None of the DMG staves actually prohibit use as a quarterstaff, despite the general rule on p.140 contemplating this.)
Yes, magic staffs are magic weapons
As Justin Time - Reinstate Monica noted earlier in a comment on nitsua60's answer, Jeremy Crawford noted in an unofficial 2016 tweet that any Magic Staff is considered a magical weapon, using the staff of frost as an example:
Can I use a Staff of Frost as Quarterstaff (the text didn't say "you can")? If yes, it does count as a magic melee weapon?
A magic staff can be used as a quarterstaff, unless its text says otherwise (see DMG, 140). It is a magic weapon.
A Magic Staff can be used as a Quarterstaff, unless its text says otherwise (see DMG, 140).
Thus, a Magic Staff is a weapon in this case. Because it is a magical item and weapon it is logical to consider him a magic weapon.
According to the Sage Advice Compendium any magic weapon can bypass resistances and immunities to damage from nonmagical attacks.
Thus, the Magic Staff can bypass resistances and immunities to damage from nonmagical attacks.
In any case, I am sure that DM should decide this question at its discretion.
Thanks to V2Blast for your edits.
Yes, it does.
Some creatures have vulnerability, resistance, or immunity to certain types of damage. Particular creatures are even resistant or immune to damage from non-magical attacks (a magical attack is an attack delivered by a spell, a magic item, or another magical source).
A magical staff is a magic item, therefore an attack delivered by a magic staff is a magical attack. Distinctions between magic weapons and magic items are totally irrelevant. An attack delivered by a magic item is a magic attack.
Yes, most magic staffs are magic weapons, unless they specify otherwise, by virtue of them being a magic item that is a staff
The term "magic weapon" isn't defined anywhere in game, so we need to break it apart. We do this by asking ourselves two questions:
- Is a magic staff a weapon?
- Is a magic staff magic?
Is a magic staff a weapon?
The answer to this is definitively yes. The DMG, under Magic Item categories, describes staffs as:
Unless a staff’s description says otherwise, a staff can be used as a quarterstaff.
So, unless the specific item's description says that that specific staff cannot be wielded as a quarterstaff, it is also a quarterstaff due to it's category.
Quarterstaffs are weapons, ergo, most magic staffs are also weapons.
But why does the DMG have a separate category for weapons in the same section?
Because of that caveat we just mentioned. Everything in the "Weapons" category is a weapon, but not all staffs are weapons. Thus a different category is required to allow for this (potential) exception for some staffs.
Is a magic staff magic?
In their Sage Advice publication, Wizard's of the Coast have given us a test we can use to determine if something is magic or not, for the purposes of the game rules. This test is:
[Our] game makes a distinction between two types of magic:
the background magic that is part of the D&D multiverse’s physics and the physiology of many D&D creatures
the concentrated magical energy that is contained in a magic item or channeled to create a spell or other focused magical effect
In D&D, the first type of magic is part of nature. It is no more dispellable than the wind. A monster like a dragon exists because of that magic-enhanced nature. The second type of magic is what the rules are concerned about. When a rule refers to something being magical, it’s referring to that second type. Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:
Is it a magic item?
Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
Is it a spell attack?
Is it fueled by the use of spell slots?
Does its description say it’s magical?
If your answer to any of those questions is yes, the feature is magical.
This is nice, and clear cut. A magic staff is (by definition) a magic item. As a result, we can answer the first question in the test with a yes, and can as a result conclude that it is indeed magic.
Magic staffs are also magic weapons (mostly)
Since most magic staffs, as long as they don't satisfy the caveat we saw earlier, are both weapons, and are magic, they are, for the purposes of the rules, magic weapons. As a result, for the purposes of overcoming resistance to attacks made by "nonmagical weapons", a magic staff would also overcome this resistance.
Yes, it does
By default any permanently enchanted weapon is magic and overcomes damage resistance against non-magical weapons, even if it doesn't have a plus. This extends to a staff used as a quarterstaff. Other magic items(EG used as an improvised weapon) are at DM's discretion.
The reason some staffs specify they can be used as a quarterstaff is to clarify they have a +1,+2, or +3 bonus when used as a weapon.