  A triplon refers to a fictitious particle that carries angular momentum $S =
1$ corresponding to the elementary excitation in a broad class of quantum
dimerized spin systems. Such systems without magnetic order have long been
studied as a testing ground for quantum properties of spins. Although triplons
have been found to play a central role in thermal and magnetic properties in
dimerized magnets with singlet correlation, a spin angular momentum flow
carried by triplons, a triplon current, has not been detected yet. Here we
report spin Seebeck effects induced by a triplon current: triplon spin Seebeck
effect, using a spin-Peierls system CuGeO$_3$. The result shows that the
heating-driven triplon transport induces spin current whose sign is positive,
opposite to the spin-wave cases in magnets. The triplon spin Seebeck effect
persists far below the spin-Peierls transition temperature, being consistent
with a theoretical calculation for triplon spin Seebeck effects.
