The victory was only the second hard court title for 25-year-old Moya, and his first on the surface in a Masters Series event.
The Spaniard has now won four titles this year, and eleven in his career, including the 1998 French Open and Masters Series Monte Carlo in 1998.
Moya also becomes the first Spaniard ever to win in Cincinnati and did so without dropping a set.
Hewitt made the early running in the first set, moving to a 3-1 lead over the top seed before being broken back for 3-3 under rapidly darkening skies.
The heavens opened at 4-4 after just 34 minutes of play and when it resumed two and a quarter hours later, Hewitt’s serve faltered again at 5-6 under the weight of Moya’s destructive forehand.
World number one Hewitt took firm control of the second set, racing to a 5-2 lead only for Moya to claw his way back with the aid of some indifferent serving from the Australian and some sustained aggression of his own.
It was that aggression which helped him save set points at 3-5 down.
In an edgy tiebreaker, the pair exchange mini breaks three times before a Hewitt double fault gave Moya a match point, which he took with a booming serve.
Moya, seeded 16th here, has been on a hot streak since the start of the season and has now won 20 of his last 22 matches.
Cincinnati marks his biggest pay day of the year so far though, earning him $392,000 in prize money.
There was more bad news for Hewitt.
His performance this week has earned him $206,000 but after a fracas with the ATP earlier in the week over his refusal to do a TV interview on Monday, Hewitt could be fined as much as half of prize money, so his week’s work could cost him $103,000.
MANHATTAN BEACH: Chanda Rubin was forced to come behind to beat defending champion Lindsay Davenport 5-7, 7-6, 6-3 in a close final at the Los Angeles Open Sunday.
Third seeded Davenport broke to serve out for the match at 5-4 in the second set, but Rubin rallied, was always ahead in the tiebreak, and sealed her victory by winning the final three games of the two hour, 19 minute match.
It was Rubin’s third final and second title of the year — she lost the Spanish Open final in May and won the Wimbledon warm-up event on the Eastbourne grass in June.—Reuters