The US singer, 38, belted out his hits “Ordinary People”, “All of Me” and “Surefire” after sitting down at a piano on the concourse.
Legend was surrounded by fans this morning during his surprise concert, many of which filmed the 10-minute set on their mobile phones.
Legend hinted at his plans ahead of his arrival on the Eurostar from Paris by tweeting: “Arriving at London @StPancrasInt on @EurostarUK. Do they still have that piano there?”
Straight after the gig, Legend was ushered through the station into a waiting car and turned down requests for selfies from fans.
For those that missed out, the performance was live streamed on promoter Live Nation’s Facebook page.
The gig was to promote his upcoming world tour, which will see him return to the UK for shows in September.
Come back soon, John!