Monaco, seeded third, bested Haas 7-5, 6-4 at Hamburg, Germany, improving to 6-9 all-time in ATP finals, including 3-1 this year after earlier clay-court victories at Houston and Vina del Mar.
Monaco is expected to be included in the world's Top 10 rankings after Sunday's victory.
Haas, a 34-year-old Hamburg native, fell to 13-10 in finals. Monaco broke his serve five times en route to the win.
18-year-old elf alleges mall Santa pinched her buttocks on the job
Texas principal bans speaking Spanish, stirs controversy