When Iron Man welcomed Loki to Instagram

Robert Downey Jr is all about Tom Hiddleston joining Instagram.

The award-winning actor posted a shot of Taylor Swift's beau from his Fourth of July vacation (you know the one, where he was wearing his I heart TS tank top) and wrote a hilarious caption to boot.

"Join me in welcoming the biggest T Stark fan of them all to Instagram!" he wrote poking a bit of good-natured fun at Hiddleston's high-profile romance with the pop princess.

Robert Downey Jr and Tom Hiddleston

The 35-year-old Brit joined the social media platform on Tuesday and posted his debut shot in his Marvel character Loki.

"He's back!" Hiddleston wrote presumably from the set of Thor: Ragnarok, which has been filming in Australia over the past several weeks.