YY Cet is a 10.5 mag semidetached variable with a 19 hr orbital period. The Wilson-Devinney program is used to simultaneously solve two new sets of UBV light curves together with preexisting photometry and single-line radial velocity measurements . The system has the lower-mass component completely filling its Roche lobe. The resulting masses are M1=1.78+/-0.19M_{sun} and M2=0.92+/-0.10M{sun}, and the radii are R1=2.08+/-0.08R{sun} and R2=1.62+/-0.06R{sun}_. Its computed distance is 534+/-28pc. Light- and velocity-curve parameters, orbital elements, and absolute dimensions are presented. A study of published TOM observations indicates that the period changed around 1999.