The short answer is that the Rage provides resistance to bludgeoning, piercing, and slashing damage, regardless of their source (magic weapon or not).
What type of damage is rage damage?
While you’re raging, the first creature you hit on each of your turns with a weapon attack takes extra damage equal to 1d6 + half your barbarian level. The extra damage is necrotic or radiant. Only the damage from this feature is necrotic or radiant.
Do barbarians have resistance to magical damage?
The exclusion of any mention of magical weapons isn’t accidental, Barbarians are resistant to bludgeoning, piercing, and slashing damage whether it’s magical or otherwise as it’s key to their ability to have staying power on the front line.
When does the Barbarian’s damage resistance from Rage apply?
The barbarian’s damage resistance does apply to damage from magical weapons and more. The answer by Pyrotechnical is supported by (then-official-but-no-longer) tweets ( 1, 2, 3) by Jeremy Crawford, answering specifically this question in April 2016:
What are the advantages of being a barbarian?
You have advantage on Strength checks and Strength saving throws. When you make a melee weapon attack using Strength, you gain a bonus to the damage roll that increases as you gain levels as a barbarian, as shown in the Rage Damage column of the Barbarian table. You have resistance to bludgeoning, piercing, and slashing damage.
Can a barbarian resist damage from a spell?
JC: Yes. Note that Crawford’s answer covers even more than what is asked in this particularly question: even the bludgeoning, piercing, and slashing damage from spells is resisted. Thanks for contributing an answer to Role-playing Games Stack Exchange!
What happens when you make a melee attack as a barbarian?
When you make a melee weapon attack using Strength, you gain a bonus to the damage roll that increases as you gain levels as a barbarian, as shown in the Rage Damage column of the Barbarian table. You have resistance to bludgeoning, piercing, and slashing damage.
