Ronchi bettered his highest T20 score of 48 set in Auckland last Saturday with an unbeaten 51 off 28 balls at the match in Wellington.
He put on 68 for the sixth wicket with Ross Taylor then remained calm as Jimmy Neesham hit a six and two fours in the last four balls of the 19th over to wrap up the win.
Ronchi said it was satisfying to repay the confidence coach Mike Hesson and captain Brendon McCullum had shown in him.
"It's nice just to perform, I guess," he said.
"I've been feeling really good but I hadn't been putting numbers on the board. It's been good to still have the faith of Brendon and Hess, and to put two good performances up in these games.
"It's just confidence - it's nice to know I can actually do it. I guess I've just got to stay consistent with it, stay nice and relaxed, and go out and play the way I play cricket."
McCullum said Ronchi's contribution had been outstanding in both T20s.
"It's been the difference in both games, his ability to come in and play as free as what he has and dominate the partnership in the last two games, and put the pressure straight back on the West Indies - it's been brilliant," he said.
"We know how good he is, and everybody else is starting to see it."
The match proved a far harder work-out than the first T20, where New Zealand cruised to an 81-run victory at Eden Park.
Chasing 160 for victory in Wellington on Wednesday, the Black Caps couldn't capitalise on some sloppy West Indies fielding early on and lost wickets regularly to sit on 5-79 as the halfway mark ticked over.
Taylor and Ronchi resuscitated the innings, although the former New Zealand captain's dismissal for 39 four balls into the 18th over kept the Kiwis under pressure until Neesham took charge.
Earlier, a whirlwind 55-run knock from wicketkeeper Denesh Ramdin boosted the visitors to 159-5 after they won the toss.
Ramdin, who combined with Andre Fletcher in a 70-run fifth-wicket partnership, was unbeaten in a 31-ball knock which included three fours and three sixes.
New Zealand beat the West Indies 2-0 in the three-match Test series and drew the five-match one-day series 2-2.
They now face India in a five-match one-day series beginning in Napier on Sunday.