The Lotto-Soudal rider, a stage winner in 2017, was perfectly positioned after a tight curve 200 metres from the line and beat Italian champion Elia Viviani (Deceuninck-Quick Step) in a short sprint.
German Pascal Ackermann (Bora-Hansgrohe), who leads the points classification, took third place.
Italian Valerio Conti of Team UAE retained the overall leader's pink jersey.
"Determination and the help of the team made me a winner today. We've been trying for seven days to get this win. I'm just so happy to win this stage," said Ewan.
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"I wanted to be first in the last corner but it was still a long way away so it worked better to get out of Ackermann's wheel. I knew I had a good kick and I went around him."
Sunday's ninth stage is a 34.8-km individual time trial from Riccione to San Marino featuring a 12.25-km climb at the end.
(Writing by Julien Pretot; Editing by Clare Fallon)
