An efficient and exact spectral integral method (SIM) for the general problems of scattering by a circular cylinder is presented, both for acoustic and electromagnetic cases. Fast Fourier transform (FFT) and convolution theorem help us to execute the algorithm efficiently with O(N log(N )) computational complexity. The Fourier coefficients of the integral kernels are computed in a semianalytical way to accomplish a fast convergence rate with spectral accuracy. Compared with the method not using the semianalytical forms, without the increase of the computing and memory abundance, it saves more than 90% sampling points to achieve about the same accuracy, and with the same number of sampling points, several to more than ten orders of magnitude more accuracy can be achieved. For example, for the tested large wavenumber case with 2 × 10 6 wavelengths on the boundary, only two sampling points per wavelength are required to achieve a relative error of less than 0.001%. More than 90% computational time is saved compared with the normal summation formula of harmonic cylinder expansions.