A planar ultra-wideband transmitarray of tightly-coupled dipoles based on a novel modular unit-cell is proposed. Specifically, modularity is achieved by using capacitive coupling to connect the dipoles with the true-time-delay lines. Therefore, this modular design does not require physical connection between the receive/transmit dipole-layers and the true-time-delay lines, thereby enabling the realization of the first (to our knowledge) planar ultra-wideband transmitarray. Also, our design reduces significantly the fabrication cost of ultra-wideband transmitar-rays. As a proof-of-concept, a 12×12 element transmitarray is designed for broadside operation, and provides a 5:1 frequency bandwidth with an active VSWR < 3 from 3 to 15 GHz.