Vertebrates with terrestrial or freshwater ancestors colonized the sea from the Early Triassic onward and became competitively dominant members of many marine ecosystems throughout the Mesozoic and Cenozoic eras. The circumstances that led to initial marine colonization have, however, received little attention. One hypothesis is that mass extinction associated with ecosystem collapse provided opportunities for clades of amphibians, reptiles, birds, and mammals to enter marine environments. Another is that competitive pressures in donor ecosystems on land and in freshwater, coupled with abundant food in nearshore marine habitats, favored marine colonization. Here we test these hypotheses by compiling all known secondarily marine amniote clades and their times of colonization. Marine amniotes are defined as animals whose diet consists primarily of marine organisms and whose locomotion includes swimming, diving, or wading in salt water. We compared the number of clades entering during recovery phases from mass extinctions with the rate of entry of clades during nonrecovery intervals of the Mesozoic and Cenozoic. We conservatively identify 69 marine colonizations by amniotes. The only recovery interval for which prior mass extinction could have been a trigger for marine entry is the Early Triassic, when four clades colonized the sea over 7 Myr, significantly above the rates at which clades entered during other intervals. High nearshore productivity was a greater enticement to colonization than was a low diversity of potential marine competitors or predators in nearshore environments of a highly competitive terrestrial or freshwater donor biota. Rates of marine entry increased during the Cenozoic, in part because of rising productivity and in part thanks to the participation of warm-blooded birds and mammals, which broadened the range of thermal environments in which initial colonization of the sea became possible.