"Saturday Night Live" host Ariana Grande and her "Wicked" co-star beau Ethan Slater looked "in love" during the NBC show's afterparty at L'Avenue at Saks, a source tells Page Six.

"Ariana and Ethan were side by side throughout the evening, looking happy and in love," said a spy.

We hear Grande, 31, also brought her close friend from her Nickelodeon days, Elizabeth Gillies, to the bash.

The pop star was also spotted hanging out with "SNL" regular Bowen Yang, who also stars in the upcoming movie version of the hit musical "Wicked."

A source tells us Grande and Slater left around 3:30a.m.

Musical guest Stevie Nicks, 76, was also making the rounds at the party.

A spy said that the Fleetwood Mac icon and "SNL" creator Lorne Michaels, 79, were "engaged in what appeared to be a long, heartfelt discussion."

They were the last two guests at the party, recounts the spy, leaving just after 4:30 A.M.

Scarlett Johansson also turned up with her husband, "Weekend Update" anchor Colin Jost.

Show alumni Maya Rudolph, Andy Samberg and Dana Carvey mingled about the room.

"Barbie" star Simu Liu, Kennedy scion Jack Schlossberg, comedian Jim Gaffigan and Haim rocker Este Haim also attended.

Earlier on the show, Grande promised she wouldn't sing or "do my impressions" -- but then went ahead and did impressions of Britney Spears, Miley Cyrus and Gwen Stefani.

Grande previously hosted "SNL" back in March 2016, when she was also the musical guest.