Leo DiCaprio Cuts Loose at Art Basel Party After Suited Speech Day Before

Leonardo DiCaprio can be buttoned-up Mr. Environment one minute, and Hollywood's favorite party boy the next -- a feat he proved yet again this weekend at Art Basel in Miami.

The actor was on hand yet again Friday for a huge bash in the Magic City, where Wayne and Cynthia Boich hosted a gaggle of stars at their private mansion ... including Leo and his girlfriend, Vittoria Ceretti, who enjoyed a smoke next to her man at one point in the evening.

Leo was in party mode, rocking his signature black ball cap and blowing out whatever he was puffin' on -- with Vittoria hitting her vape. Quite the change from how he looked Thursday night, when he was front and center for a speech at an environmental event.

The Re:Wild get-together where Leo talked featured him in eco mode ... where he was as polished as can be and looking real sharp. 24 hours later, he's letting it all hang out!

Anyway, the mansion party on Friday was a who's-who of stars ... Tom Brady was there, but so was Sean Penn -- who actually hugged it out with Leo at one point -- as seen here in a photo obtained by TMZ.

Sean was mingling with folks inside as well ... fresh on the heels of a performance by Snoop Dogg, who'd performed for the rich and famous crowd not long before. Speaking of SD ... he was also chitchatting in the crowd after doing his thing on the mic, which was awesome.

Snoop performed a few of his songs ... and he definitely had everybody dancing along.

It's just the latest Art Basel stop for Leo, who's been in town since earlier this week. Something tells us we're gonna be seeing a lot more of him through Sunday too.