The third benefit of the [Sentinel](https://www.dndbeyond.com/characters/feats/sentinel) feat (PHB, p. 169-170) says:

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> When a creature within 5 feet of you makes an attack against a target other than you, you can use your reaction to make a melee weapon attack against the attacking creature. 
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The "reach" property of weapons applies to normal attacks, bonus attacks, and opportunity attacks, but this Sentinel attack is not explicitly any of those. Can I make this attack with my reaction against someone 10 feet away who makes an attack against a target other than me if I have a weapon with a 10-foot reach?