The singer took to Instagram to share of a photo of the two embracing in a loving hug, calling Momoa, who turned 40 Thursday, his "brother."
Kravitz's ex-wife, actress Lisa Bonet, with whom he shares daughter and actress Zoë Kravitz, married the "Aquaman" star in 2017.
"Happy birthday brother," Kravitz captioned the post to his blended family member. "Love and respect always. Ohana ‘til the end," including the Hawaiian word meaning family.
In a recent interview with The Sunday Times, the rock icon opened up about the family's unique dynamic.
"Our families are blended," Kravitz said. "I love her husband – he’s like a brother to me – and I love the kids. It’s beautiful, but it takes work."
Momoa, 39, and Bonet began dating in 2005 and have two children together: 12-year-old Lola and 10-year-old Nakoa-Wolf.
The superhero actor celebrated his 40th birthday at a surprise party, sharing a behind-the-scenes video Sunday of the gathering.
When Momoa arrived to party, organized by his friends, they sang "Happy Birthday," serenading him with an extravagant cake featuring an edible version of his likeness surfing, playing guitar and hanging off of a rock.
While Bonet wasn't at the party in person, Momoa brought her around over video call, showing her the slideshow his friends made for him and exclaiming that his friends "(expletive) got me" with the surprise.
Contributing: Hannah Yasharoff
This article originally appeared on USA TODAY: Jason Momoa gets 40th birthday message from 'brother' Lenny Kravitz