The seventh seed was made to work hard by the 23-year-old Pole but Rosol's experience won through to ensure a 3-6 7-6 (7-3) 7-5 victory.
Rosol, appearing in his third singles final of the year, had beaten fellow seeds Pablo Andujar and Lu Yen-hsun to make the final but was behind early on in North Carolina.
While Janowicz swiftly moved into the lead after a 34-minute opening set, Rosol drew level on a tie-break before taking advantage of his opponent's missed match-point chances in the third to see out the match.
The title is the 29-year-old's first on Tour since clinching the Nastase Tiriac Trophy in Bucharest last year.
Despite dropping serve in the third game of the opening set, Janowicz hit back impressively to break Rosol at the next two attempts to give himself a 4-2 lead.
The world number 52 then kept his cool to serve out for the set before the pair matched one another game for game throughout the second.
Rosol required patience to tie the match - saving six break points and winning 78 per cent of first service points in the second set to draw level.
The deciding set was just as closely fought and saw Janowicz waste two match-point opportunities in the 10th game.
Rosol took full advantage to break and move 6-5 ahead before serving out for the championship - beating Janowicz for the second time in his career and denying the Pole a first Tour-level singles title.