To keep pace with the demands in optical communications, electro-optic modulators should feature large bandwidths, operate across all telecommunication windows, offer a small footprint, and allow for CMOS-compatible fabrication to keep costs low(1). Here, we demonstrate a new ultra-compact plasmonic phase modulator based on the Pockels effect in a nonlinear polymer. The device has a length of only 29 mu m and operates at 40 Gbit s(-1). Its modulation frequency response is flat up to 65 GHz and beyond. The modulator has been tested to work across a 120-nm-wide wavelength range centred at 1,550 nm, and is expected to work beyond this range. Its operation has been verified for temperatures up to 85 degrees C and it is easy to fabricate. To the best of our knowledge, this is the most compact high-speed phase modulator demonstrated to date.