Mangrove trees provide environmental buffering for animals by reducing daytime water thermal maxima. Shade from Rhizophora mangle trees reduces thermal stress for the mosquitofish Gambusia affinis. Data were collected from mangrove forests in Quintana Roo, Mexico, at 2 sites: (1) Soliman Bay, a mangrove forest decoupled from direct ocean water, and (2) La Lunita with semi-direct access to ocean water. During cooler morning hours at Soliman Bay, large numbers of mosquitofish foraged in open channels. At the same time, few mosquitofish utilised shaded areas within the R. mangle prop roots. When channels exceeded water temperatures above 38°C, mosquitofish migrated into the shaded areas provided by R. mangle trees. Channel water reached a daytime maximum temperature of 46°C, while daytime-shaded R. mangle areas remained 6.2°C cooler. Temperature rise and abundance shifts were not found in La Lunita, which has water temperatures of 27 to 28°C throughout the day. Size distributions between both localities showed mosquitofish caught in Soliman Bay to be >10% smaller than the mosquitofish from La Lunita, which had a greater abundance of mature adults. In Soliman Bay, mosquitofish were small, and the thermal stresses may impose serious developmental impacts that would be reduced by occupying water shaded by R. mangle. Refuge from R. mangle provides environmental buffering for fish, and future increasing water temperatures may reveal a greater use of this little-known mechanism. This study provides an example of a mechanism whereby mangroves support intertidal biodiversity through environmental buffering.