The reproduction and population dynamics of Portunus trituberculatus were investigated in 5402 specimens caught on the western coast of Korea, Yellow Sea, during three consecutive years between September 2003 and August 2006. The overall ratio of males to females was 1 : 1.08, although the sex ratio varied slightly from month to month and year to year. Over 90% of the females were in the maturing or mature stages in March-May, the gonadosomatic index was highest in April-May, and ovigerous females peaked in July. These results indicate that the main breeding season was between April and July. The frequency of crab larvae was highest in August, which is clearly supported by the breeding season. At sexual maturity, 50% of females had an estimated carapace width (CW) of 13.12 cm. The relationship between the number of eggs per brood (EN) and CW was ln EN = 3.04 ln CW + 6.83 for non-eyed eggs, and ln EN = 3.31 ln CW + 6.03 for eyed eggs. No brood loss occurred during the incubation period. The reproductive output was 0.34. The growth performance index showed that females grew faster and reached a larger size at the same age than males (L∞ = 22.1 mm CW and K = 0.66 yr–1 for females, and L∞ = 20.4 mm CW and K = 0.72 yr–1 for males).