The Mad Men actress was previously married to actor Geoffrey Arend between 2009 and 2019.
Kat Dennings and Andrew W.K. announced their proposal in May last year.
The couple went public with its romance in early May.
Kat Dennings has shared three photos of her stunning diamond engagement ring on Instagram.
Dennings made her debut as Darcy Lewis in the 2011 Marvel blockbuster.