New Zealand took the series 3-2, clinching it in Game 4 after winning Game 2 by five wickets and Game 3 by six wickets in a comeback following West Indies' seven-wicket victory in the opening game.
Hetmyer's fifth ODI half-century and his partnership of 95 with Sherfane Rutherford (61) allowed the West Indies to reach 8-269 in reply to New Zealand's 9-268 with 15 balls remaining. All five matches in the series were won by the team batting second.
When Hetmyer and Rutherford were together in a fifth-wicket partnership that built on the foundation set by Shai Hope (39) and Keacy Carty (20), who put on 59 for the third wicket, the Windies looked like winning with ease.
But it proved to be a seesawing encounter. Rutherford was out when the hosts were 5-218, needing 41 to win with almost 10 overs remaining, and then Gudakesh Motie fell at 6-239.
When Jacob Duffy dismissed Matthew Forde and Alzarri Joseph with consecutive deliveries in the 45th over, and with 21 runs still needed for a West Indies win, the complexion of the match changed again.
New Zealand captain Mitchell Santner made the unusual decision to take left-arm spinner Jayden Lennox out of the attack after he had taken 3-35 from nine overs. Hetmeyer hit two sixes off Duffy in the 47th over and West Indies got home with more than two overs to spare.
Lennox took 14 wickets at an average of 13.6 and was voted player of the series.
"I wasn't really thinking about getting a half-century to be honest, I was just thinking about how ... I can make sure the team crosses the line," Hetmeyer said.
"Every time a new batter came out I was just like 'make sure you're there at the end.' I'm just happy that I could be there to make sure we crossed the line."
New Zealand appeared to be heading for a much bigger total when they were 3-181 in the 35th over and 5-230 with five overs to go.
Will Young made 56, Tom Latham made 69 and Nick Kelly scored 64 to drive the innings. But the finish was missing against tight bowling, especially from Alzarri Joseph, who took 2-38 from his 10 overs.
