Can you use Magic Weapon spell on an Improvised weapon?

Question

Tavern Brawler feat states that:

You are proficient with improvised weapons and unarmed strikes.

And the rules for improvised weapons are:

In many cases, an improvised weapon is similar to an actual weapon and can be treated as such. For example, a table leg is akin to a club. At the DM’s option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus. An object that bears no resemblance to a weapon deals 1d4 damage.

So can you use Magic Weapon spell on an Improvised weapon?

Answer 1

This depends on DM ruling.

The rules don't explicitly cover this case, so there are two, equally-valid ways to rule this.

The first is that improvised weapons don't count as weapons. This ruling is supported mainly by a few parts of the Improvised Weapons description (page 147, PHB). It says:

Sometimes characters don't have their weapons and have to attack with whatever is dose at hand... In many cases, an improvised weapon is similar to an actual weapon...

These line heavily imply that, while improvised weapons often act like weapons, they actually aren't. Since magic weapon specifies that it can only target weapons, by this ruling improvised weapons can't be made magic.

This interpretation is supported by Jeremy Crawford, who tweets:

Weapon Bond works with a bona fide weapon ("Behold, my sword!"), not an improvised weapon ("Look, a stool!").

The second interpretation is that, since the phrase "improvised weapons" contains the word "weapon", then improvised weapons must be weapons. With this interpretation, anything that can be done to a weapon can also be done with an improvised weapon.

Both of these interpretations are valid, and based on valid rules reasoning, so this is going to be dependent on your DM.

Answer 2

Yes, if you can do it while it is a weapon

There are really two questions here. The first is whether an improvised weapon counts as a weapon. That is easily answered by the rules on improvised weapons (emphases mine):

Often, an improvised weapon is similar to an actual weapon and can be treated as such. For example, a table leg is akin to a club. At the DM's option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus.

These rules make it clear that while an improvised weapon is not a weapon in an existential sense, it can be treated as one in a functional sense. Thus, for the purpose of other rules that interact with weapons, including the magic weapon spell, improvised weapons will count as weapons.

The second question is more difficult - when is an improvised weapon a weapon? Are all the table legs in the world sitting there as weapons, or do they become weapons only when someone breaks them off to use in a fight?¹ This 'timing' question, when does an object become a weapon, is irrelevant to a combat: what matters is that the table leg is a weapon at the moment of an attack, and whether it is one before or after is inconsequential. The timing is, however, essential to interact with the spell, because the spellcasting rules require that something be a legitimate target at the time the spell is cast.

Here the RAW don't help us out. Consider a situation in which two hostile parties have agreed to parlay at a neutral site with no weapons permitted for those in attendance, but I visit the spot first to enchant an improvised weapon. Can I cast magic weapon (duration: 1 hour) on the table leg before it is removed from the table? Can I cast the spell while I or my ally is wielding the table leg but before combat has started or before my foe is in sight?

Although the improvised weapon rules themselves don't help here, a tweet from Jeremy Crawford² provides some perspective on RAI:

An improvised weapon is, indeed, a weapon, but only the moment it's used as such. A chair/shield/etc isn't a weapon otherwise.

If a DM accepts this unofficial guidance, then the magic weapon spell can be used on an improvised weapon, but only at the "moment" it is used as a weapon, since outside this moment it would not count as one. This still leaves some latitude for DM interpretation of what a "moment" is, though. Given the 'simultaneous nature' of a combat round, I personally would be inclined to permit casting on anything the caster or an ally had in hand while they were in the initiative order of a combat round. A more conservative DM might restrict casting to during the attack sequence itself, which would be hard to pull off on something wielded by an ally, given that the spell's casting time is a bonus action and casting can thus only be performed on the caster's own turn.

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¹ If a table leg was alone in the forest, and no one was there to wield it, would it be a weapon?

² There are different schools of thought on Crawford's unofficial tweets. I tend to ignore them when they are unrelated or contradictory to clear RAW. In cases such as this, when RAW is unclear, I think they can be helpful in understanding RAI.

Answer 3

RAW, yes

Magic weapon only stipulates that the target is

A nonmagical weapon

Improvised Weapons is a category of weapon properties and only weapons can have weapon properties. Therefore, RAW, an improvised weapon qualifies.

By definition,

An Improvised Weapon includes any object you can wield in one or two hands

So, RAW, any object that you can so wield may be the target of Magic Weapon.

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You can read through some more of the nuances in @NautArch's explanation here on why Improvised Weapons are weapons. For comparison, check @EddyMage's rationale about Natural Weapons as weapons.