New high-resolution spectra of the short-period (P similar to 1.76 days) young detached binary LT CMa are reported for the first time. By combining the results from an analysis of new radial-velocity curves and published light curves, we determine values for the masses, radii, and temperatures as follows: M-1 = 5.59 (0.20)M-circle dot, R-1 = 3.56 (0.07) R-circle dot, and T-eff1 = 17000 (500) K for the primary and M-2 = 3.36 (0.14)M-circle dot, R-2 = 2.04 (0.05) R-circle dot, and T-eff2 = 13140 (800) K for the secondary. Static absorbtion features apart from those coming from the close binary components were detected in the several spectral regions. If these absorbtion features are from a third star, as the light-curve solutions support, its radial velocity is measured to be RV3 = 70 (8) km s(-1) The orbit of the binary system has been proved to be eccentric (e = 0.059) and thus apsidal motion exists. The estimated linear advance in longitude of periastron corresponds to an apsidal motion of U = 69 +/- 5 yr for the system. The average internal structure constant, log k(2,obs) = -2.53 of LT CMa, is found to be smaller than its theoretical value of log k(2,theo) = -2.22, suggesting that the stars would have a more central concentration in mass. The photometric distance of LT CMa (d = 535 +/- 45 pc) is found to be much smaller than the distance of the CMa OB1 association (1150 pc), which rules out membership. A comparison with current stellar evolution models for solar metallicity indicates that LT CMa (35 Myr) is much older than the CMa OB1 association (3 Myr), confirming that LT CMa is not a member of CMa OB1. Kinematical and dynamical analyses indicate that LT CMa is orbiting the Galaxy in a circular orbit, and belongs to the young thin-disk population.