The 10-time Grammy winner tweeted that he was about to get off the Eurostar from Paris.
Within 20 minutes his post had drawn a big crowd who got to hear an eight-minute performance, including Ordinary People.
Afterwards Legend didn't have time for fan selfies as he was escorted away to an unknown location.
This is the tweet that got things started.
Newspaper reporters and other media weren't far behind.
His PR team were ready and poised.
And fans who got down there in time were pretty pleased.
But there were others feeling pretty gutted to miss out.
John Legend is promoting his forthcoming world tour. He'll be back playing planned gigs in the UK in September.