Dating from 1556, the Templo Santo Domingo de Guzmán (Av. Rosario Castellanos at Calle Central Ote., 10 a.m.–7 p.m. daily) is the most imposing building on the central plaza [1]. It was the last Dominican church built in Chiapas [2] and is unique for its severity—other than the mustard-yellow paint job, the octagonal choir window is the only decoration on its facade.
Be sure to take a good look at the bell tower; the Mudejar-style arcading was only recently discovered under its stucco surface, a major discovery for early colonial architecture buffs.
The convent, unfortunately, was demolished; the brick building that stands in its place houses the Centro Cultural Rosario Castellanos (central plaza, Av. Rosario Castellanos at 1a Calle Sur Ote., tel. 963/632-0624, 9 a.m.–8 p.m. Mon.–Fri., 10 a.m.–5 p.m. Sat.–Sun., free).
Links:
[1] http://www.moon.com/destinations/chiapas/the-lakes-region/comitan/sights/el-zocalo
[2] http://www.moon.com/destinations/chiapas