The pop star, dressed in black shorts and a sweatshirt, appeared to be keen to show his love for the model as he hopped out of the couple's black SUV with his guitar, sat by a fountain and started playing his song "Coldwater".
Fans quickly gathered around the couple and many posted footage and photos on social media. Some suggested that Bieber was asking for song requests.
The loved-up couple arrived in London at the beginning of the week, and Justin and Hailey haven't been shy about showing off their love - kissing and holding hands in public.
On Friday, September 14, Hailey took to Twitter to dismiss reports suggesting the couple had already wed.
"I understand where the speculation is coming from, but I'm not married yet!" the model wrote.
And her uncle, actor Alec Baldwin, started the rumor mill turning all over again on Monday night at the Emmys when he told reporters he was sure his niece was already Mrs. Bieber.