Lewis, playing his first ODI since 2021, hit a straight six to bring up the winning runs and his fifth ODI century off 61 balls in Pallekele on Saturday. The opener played through even after rolling his left ankle on 51.
The West Indies reached 2-196 in reply to Sri Lanka's 3-156 after a five-hour rain delay reduced the match to 23 overs per side at Pallekele International Stadium.
Pathum Nissanka, dropped before he scored and on 17, and Avishka Fernando, also dropped, opened with an 81-run stand in 17 overs for Sri Lanka followed by the rain delay.
When play resumed, Kusal Mendis hit the first four deliveries to the boundary.
Nissanka reached his 15th ODI half-century off 58 balls and was run out for 56.
Mendis went on to a 19-ball half-century and was dropped twice before finishing 56 not out after nine boundaries and a six over midwicket.
The West Indies lost their first wicket on 36 but Lewis and captain Shai Hope combined for 72 runs.
When Hope was caught for 22, Lewis was joined by Sherfane Rutherford and they saw West Indies home by eight wickets with one over to spare.
Rutherford took just 26 deliveries for his fourth ODI half-century, an unbeaten 50 including three sixes and four boundaries.
He also scored 74 and 80 in the series but Sri Lanka captain Charith Asalanka was named player of the series for scoring 145 runs and taking two wickets.
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