Lady Gaga wears a pretty velvet dress as she cosies up to fiancé Michael Polansky at The Joker: Folie à Deux afterparty in Los Angeles

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Lady Gaga showed off her figure while partying away with her fiancé Michael Polansky in Los Angeles on Monday.

The singer and actress, 38, looked nothing short of sensational in a figure-hugging black velvet dress as she let loose after walking the red carpet earlier that evening.

The entertainer managed a quick costume change for her night out following the US premiere of The Joker: Folie a Deux in California .

Lady Gaga, whose real name is Stefani Germanotta, opted for a slinky dress with spaghetti straps in contrast to her earlier bold look.

The star went for soft make-up and channelled her character Harley Quinn as she kept her bright red locks in loose waves.

The Star Is Born actress was spotted smiling with Joker writer and director Todd Phillips and his wife Alexandra Kravetz at the star-studded after party.

Lady Gaga was also seen cosying up to her fiancé Michael as she posed alongside actor Brendan Gleeson and director Baz Luhrmann .

The star and her partner seemed in good spirits and couldn't help but smiles as they wrapped their arms around each other.

The singer's partner opted for a casual look for the event, wearing a navy blue shirt and matching cap.

Gaga stars in the latest Joker installment alongside Joaquin Phoenix , 49, who joined her on the red carpet earlier that evening.

The singer and actress plays Harley opposite Joaquin's Joker in the psychological thriller.

The movie serves as a follow-up to Joaquin's 2019 box office hit, which was simply titled Joker.

The musical is due in theatres on October 4, following an October 2 international release.

And for Monday evening's appearance in Hollywood at TCL Chinese Theatre, La dy Gaga turned heads in a black gown with billowing, satin, short sleeves.

She rocked bleached eyebrows and sported a red wig as she graced the red carpet solo.

One month before the sequel's release date, Joaquin praised Lady Gaga for her 'impressive' preparation for the feature film.

During a Venice Film Festival press conference in which he discussed shedding pounds for Joker, he added: 'Stefani also lost a lot of weight.

'I remember when I first met you in rehearsals and then you went away for like a month and then you came back, you lost a lot of weight.'

He continued, 'It was really impressive. It seemed very difficult.'

The singer agreed, 'I think we transformed into our characters over a period of time and we continued to hone in every kind of detail.'

She plays a music therapist at Arkham Asylum in The Joker and said: 'We rehearsed the waltz in the film for two months which kind of falls apart.'

Joaquin noted about the musical aspect: 'Part of the joy for me was to take standards and find a way that was specific to the characters.

'That was part of what was interesting for us. We started off with references like Frank Sinatra and Sammy Davis Jr and we started to emulate that sound.'

Joaquin attended the red carpet event in a sleek, tailored black suit with white accents.

He looked spiffy in the look, which he teamed with white socks and a button-up white shirt with a thin black tie.

The ensemble was rounded out with shiny, black lace-up dress shoes and black-rimmed eyeglasses with lightly tinted sepia lenses.

The Walk the Line star's graying hair was neatly cut with the sides short and longer pieces at the top.

He was typically handsome with a full, closely trimmed grizzled beard.

Rooney Mara, who has long been in a romantic relationship with Joaquin, was present at Monday's premiere as she supported her beau.

The mother-of-one, who shares four-year-old son River with the actor, wore a black dress with subtle pleats along its A-line skirt.

The sleeveless look boasted a strappy neckline and the actress added closed-toe black heels.

The brunette beauty parted her hair down the middle and pulled the locks back into a neat updo.

Joker, earned more than $1 billion at the global box office and remains the highest-grossing R-rated film of all time.

The second installment sees Arthur Fleck institutionalised at Arkham Asylum, awaiting trial for his crimes as Joker.

'While struggling with his dual identity, Arthur not only stumbles upon true love, but also finds the music that's always been inside him,' a press release shared.

The Los Angeles premiere comes nearly one week after Lady Gaga announced her album Harlequin, which is a companion album to the movie.

It features a 13-song track list including covers like That's Life, Oh, When the Saints, and Smile.