The Pole, who narrowly missed out on a place in Europe’s triumphant Ryder Cup squad, finished 16 under par to win by one stroke from Germany's Matti Schmid at Real Club de Golf Sotogrande in San Roque.
Richard Mansell was a shot further back in third after a tense final round played in changeable weather conditions and brought forward to earlier in the day due to the threat of storms.
Asked for his feelings afterwards, Meronk said: “Relieved, to be honest.
“It was a tough tournament, tough weather, didn’t have my best golf on the first day so to win the tournament is very special. I’m over the moon.”
Schmid did not produce the fireworks of his previous two rounds, having hit a five-under-par 67 on Friday and topped it with 65 on Moving Day.
But after going out in level par he holed from nine feet for birdie at the tenth and chipped close at the 12th to set up another.
Meronk was two over for the first three holes but responded in style with an eagle at the sixth and holed out from 120 yards for another at the ninth to go out in 34.
A 42-foot birdie putt at the tenth took him within one of the lead held by Schmid and another long eagle attempt at the 12th went close to set up a tap-in birdie.
The German dropped a shot at the 13th but responded by matching Meronk’s birdie at the next to keep his nose in front.
The Pole’s nerveless birdie at the 17th put the pressure on Schmid, who missed his par putt back on 16 as the lead swung to Meronk.
Schmid parred the par-three 17th and was left needing a birdie at the last, where Meronk made par to finish 16 under.
The German was just unable to hole a 40-foot birdie putt on the last, which would have forced a play-off, leaving Meronk to celebrate.
“I didn’t get off to a hot start, I was trying to talk to myself, stay positive," he said. "I was hitting good shots on the range in the morning so I knew I had the game.
“The eagle on the sixth gave me some momentum and then I believed and stayed super focused until the last putt, which I’m very proud of.
“That was the first time I hit the (ninth) fairway this week and just had a good number for my 54-degree (wedge), was hitting that shot yesterday on the range.
“Just hit a perfect shot, little draw, one bounce and in which was a great bonus for me and had me going for the back nine.”
Mansell, playing with Meronk, eagled the sixth after hitting a magnificent second shot to four feet, and another superb approach to the tenth set up birdie before he added another at the 12th.
He three-putted the 17th for an untimely first bogey of the day before a closing par left him on his own in third.
Jeff Winther was 12 under with Marcel Siem and Jeong weon Ko a shot further back, the latter putting himself in position for a 2024 card in the process. Defending champion Adrian Otaegui, playing in the final group with Schmid and Winther, finished tied for ninth on 10 under.