We present a weak-lensing analysis of the high-redshift cluster Cl 1604+ 4304. At z= 0.90, this is the highest redshift cluster yet detected with weak lensing. It is also one of a sample of high-redshift, optically selected clusters whose X-ray temperatures are lower than expected based on their velocity dispersions. Both the gas temperature and galaxy velocity dispersion are proxies for its mass, which can be determined more directly by a lensing analysis. Modeling the cluster as a singular isothermal sphere, we find that the mass contained within projected radius R is (3.69±1.47)[R/(500 kpc)]× 10 14 M⊙. This corresponds to an inferred velocity dispersion of 1004±199 km s-1, which agrees well with the velocity dispersion of 989 km s-1 recently measured by Gal & Lubin. These numbers are higher than the 575 km s-1 inferred from Cl 1604+ 4304's X-ray temperature; however, all three velocity dispersion estimates are consistent within∼ 1.9 σしぐま.