All attendees looked suitably glamorous in black tie and floor-length gowns.
Dressed in red, Theresa also looked the part – but her choice of outfit set Twitter alight thanks to the incredibly high slit at the back of her dress.
From the front, the wine red dress looked conservative, but as soon as Theresa turned around or walked forward, the surprising split could be seen.
As she strode forward to greet Donald, acres of her legs were on show.
Hordes of commenters took to Twitter to share their surprise, and one wrote: “@theresa_may split in back of red dress , rather too high !! Nearly saw her underwear!!”
Another shared: “I think @theresa_may should have had a rethink about her choice of dress tonight. Going up those stairs was slightly embarrassing. #TrumpVisitUK”.
Some felt the dress was not right for the occasion, and one said: “Theresa May going up the stairs in that dress is a travesty!! I’m trying to digest the utterly unsuitable dress our PM is wearing to receive Trump right now”.
One asked simply: “What’s going on with the split up the rear of @theresa_may dress?? Is it meant to be that high?”
And another felt the colour was perhaps misguided, questioning: “Shouldn’t Theresa May be wearing a blue dress”.
Not everyone was negative about Theresa’s attire, with one writing: “How high is that slit on the back of the dress!! YOU GO GURRRL!”
Another gave lukewarm praise, and said: “Afraid to say though that I like Theresa May’s dress, even though it’s basically a bridesmaid’s dress when they’re allowed to pick their own styles but designated a colour”.
Giving the PM a boost of confidence, one wrote: “@theresa_may looks stunning in that red dress!! Yas girl !!”
Theresa May was not the only woman to command attention tonight, with First Lady Melania Trump looking radiant in a J Mendel buttercup yellow gown.
Melania’s gown was perfect for a night at a Palace, and the dress featured yards of yellow chiffon.
A criss-cross design came off her shoulders, with a cape hanging from the back of the dress.
A pair of court shoes in matching yellow could be seen peeping out underneath the hem of her gown.
Melania wore her hair in a sweeping side-parting, curled loosely and hanging over her shoulders.
Her huge diamond engagement ring coud be seen sparkling on her finger, but she chose to forgo all other jewellery for her first night out in the UK.
The mum-of-one als kept her makeup understated, wearing pink lipstick and heavy mascara, but nothing more complicated.