A new process for fabricating niobium rf cavities for use in beam separators and linear accelerators is described. The best performance to date for a cavity made by this process was obtained with a TE011mode cavity, operating at 11.2 GHz at a peak magnetic field of ∼500 G, for whichQ≈2×1010. The principle advantages of the process described over other methods of fabricating niobium rf cavities are the following. First, a multiplicity of integrally connected cavities can be fabricated in a single joint free section. Second, the finished niobium cavities have a very high‐quality surface finish which is produced without the need of mechanical or chemical polishing the cavities after fabrication. Third, the niobium in the completed cavity is ultrapure and thus has an extremely low normal state resistance at liquid helium temperatures. Fourth, a high thermal conductivity between the niobium on the inside of the cavity and the surrounding liquid helium bath can be obtained when the niobium is supported with porous tungsten.