Relativistic jets are streams of plasma moving at appreciable fractions of the speed of light. They have been observed from stellar-mass black holes (similar to 3 to 20 solar masses, M-circle dot) as well as supermassive black holes (similar to 10(6) to 10(9) M-circle dot) found in the centers of most galaxies. Jets should also be produced by intermediate-mass black holes (similar to 10(2) to 10(5) M-circle dot), although evidence for this third class of black hole has, until recently, been weak. We report the detection of transient radio emission at the location of the intermediate-mass black hole candidate ESO 243-49 HLX-1, which is consistent with a discrete jet ejection event. These observations also allow us to refine the mass estimate of the black hole to be between similar to 9 x 10(3) M-circle dot and similar to 9 x 10(4) M-circle dot.