The Polearm Master feat (PHB, p. 168) says:

While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.

The War Caster feat (PHB, p. 170) says:

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

On this very topic, Mearls has said it requires an attack with the same weapon that's granting you the trigger:

Q: Do Polearm Master and War Caster combine to allow a magic user to make a spell opportunity attack when they enter reach?
A: No - polearm master applies only if you use the weapons it lists to make the attack

Another question on this very topic: Which is likely to provide more chances to opportunity attack, Polearm Master or Sentinel (with War Caster)?

But since then, we now have spells like booming blade (SCAG, p. 142):

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails. On a hit, the target suffers the attack’s normal effects, and it becomes sheathed in booming energy until the start of your next turn. If the target willingly moves before then, it immediately takes 1d8 thunder damage, and the spell ends.

So, with the addition of booming blade, if I have a quarterstaff + War Caster + Polearm Master, could I use my reaction to cast booming blade if someone steps into my reach?

  • \$\begingroup\$ Is your question specifically about using Booming Blade in this way? Also, technically, the description of Polearm Master just says, "when they enter your reach" - it has no mention of "the reach you have with that weapon", though that reflects the intent as shown by your Mearls quote. \$\endgroup\$ – V2Blast Aug 9 '18 at 23:38
  • \$\begingroup\$ Related: War Caster, a whip and a dagger, and provoking Opportunity Attacks \$\endgroup\$ – V2Blast Aug 9 '18 at 23:55
  • \$\begingroup\$ @Miniman: Nope. That part of the Polearm Master feat was not changed in errata (my portion is quoted from the latest version on DNDBeyond, and I checked the latest PHB errata as well). The description of the "reach" property was updated to mention that it applies to opportunity attacks as well, however. \$\endgroup\$ – V2Blast Aug 9 '18 at 23:58
  • \$\begingroup\$ @V2Blast My bad, I must have misremembered the change to reach. \$\endgroup\$ – Miniman Aug 10 '18 at 0:17
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    \$\begingroup\$ "Mearls [saying] it requires an attack with the same weapon that's granting you the trigger" is not a rule on itself. Do you have a reason that this would be the case besides one person's opinion? I may or may not agree, but it's definitely debatable. \$\endgroup\$ – David Coffron Aug 10 '18 at 1:01

RAW, it appears that using Booming Blade with Polearm Master and War Caster would work, but only with a Quarterstaff.

  • Polearm Master causes an enemy to provoke an Opportunity Attack; not a "melee weapon attack" which would preclude spells.

  • War Caster applies to Opportunity Attacks triggered by enemy movement; not "an enemy moving out of your reach", which would preclude Polearm Master's effect.

  • Booming Blade has a range of 5ft, targets a single enemy, and uses the weapon that allowed Polearm Master to trigger. This satisfies all requirements of both feats, as well as Mearls' answer on Twitter.

  • Finally, this only works with a Quarterstaff because other polearms viable with Polearm Master have a range of 10ft. When a creature enters that range and triggers the feat's Opportunity Attack, they would still be out of range of Booming Blade.

This appears to be implicitly confirmed by this Twitter exchange with Mike Mearls about whether Spell Sniper extends Booming Blade (it does, according to Mearls).

  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – mxyzplk Apr 23 at 21:13

Yes, the combination "quarterstaff + War Caster + Polearm Master" allows you to cast Booming Blade when someone enters your reach.

A comment by Mike Mearls from 2014 states that you are required to make the opportunity attack with an actual polearm - wielding a polearm in one hand doesn't allow you to make the polearm master opportunity attack with a non-polearm in your other hand.

No - polearm master applies only if you use the weapons it lists to make the attack.

In 2016, he came to the conclusion that combining Polearm Master, War Caster and Spell Sniper allows casting Booming Blade when someone gets within 10 ft. of you. This fits within the view that an attack with the polearm listed weapon would need to be part of the spell for it to work.
Note that Mearls rulings aren't considered official, only Jeremy Crawford's are, and RAW, you don't have to use a polearm for the attack.

Anyhow, without the feat Spell Sniper or another feature that increases Booming Blade's range to 10ft (such as a Sorcerer's Distant Spell metamagic option), you can only combine Booming Blade with a quarterstaff if you want the polearm master opportunity attack.
This is because the feat says "You can keep your enemies at bay with reach weapons". This implies that letting them get close to 5ft, even though your reach is 10ft, is against the wording of the spell since you're not keeping them at bay with a reach weapon in that case.

Using Spell Sniper allows you to extend the range of Booming Blade to 10 ft, which allows you to satisfy both requirements. You are keeping them at bay, and you are using a reach weapon listed in the feat.

  • \$\begingroup\$ I think your heading is a little misleading as you seem to be trying to say 'yes, but a feat is needed'. I would also provide the cited quote in your post in case of link rot. \$\endgroup\$ – Pyrotechnical Aug 10 '18 at 1:57
  • \$\begingroup\$ @Pyrotechnical My answer is only misleading after you changed the question. The OP has not confirmed that their question is specific to Booming Blade. \$\endgroup\$ – Luke Aug 10 '18 at 2:11
  • \$\begingroup\$ the title was misleading, the context of the querent's post focused specifically upon Booming Blade. \$\endgroup\$ – Pyrotechnical Aug 10 '18 at 2:15
  • \$\begingroup\$ No, an example of booming blade was given. The same question could currently apply for Green-Flame Blade. And in future a possible spell that includes a weapon attack that already has a 10ft reach. \$\endgroup\$ – Luke Aug 10 '18 at 2:17
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    \$\begingroup\$ Verbatim: "So, with the addition of Booming Blade, if I have a quarterstaff + War Caster + Polearm Master, could I use my reaction to cast Booming Blade if someone steps into my reach?" \$\endgroup\$ – Pyrotechnical Aug 10 '18 at 2:24

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