This article presents a novel low‐profile planar microstrip transmitarray antenna with high efficiency. First, a double‐layer planar microstrip transmitarray element is designed. The element consists of only one single substrate layer and two metallic layers, and two same modified Jerusalem cross‐shaped patches are etched on two sides of the substrate. These two patch layers are also connected by four specially designed metallic vias. The transmitarray element achieves more than 360° transmission phases with more than −3‐dB transmission amplitudes. Then, a low‐profile X‐band 145‐element circular‐aperture microstrip transmitarray antenna with the standard pyramidal horn feed is designed, fabricated and measured. The thickness of the microstrip transmitarray is 0.14λ0, where λ0 is the wavelength corresponding to the center frequency 10.6 GHz. Measured results indicate that gain of the transmitarray antenna at the center frequency 10.6 GHz is 25.4 dBi, its radiation efficiency is 53.8%, and its 1‐dB gain bandwidth is 7.8%.