  The Griffiths phase-like features and the spin-phonon coupling effects
observed in Tb$_2$NiMnO$_6$ are reported. The double perovskite compound
crystallizes in monoclinic $P2_1/n$ space group and exhibits a magnetic phase
transition at $T_c \sim$ 111 K as an abrupt change in magnetization. A negative
deviation from ideal Curie-Weiss law exhibited by 1/$\chi(T)$ curves and
less-than-unity susceptibility exponents from the power-law analysis of inverse
susceptibility are reminiscent of Griffiths phase-like features. Arrott plots
derived from magnetization isotherms support the inhomogeneous nature of
magnetism in this material. The observed effects originate from
antiferromagnetic interactions which arise from inherent disorder in the
system. Raman scattering experiments display no magnetic-order-induced phonon
renormalization below $T_c$ in Tb$_2$NiMnO$_6$ which is different from the
results observed in other double perovskites and is correlated to the smaller
size of the rare earth. The temperature evolution of full-width-at-half-maximum
for the {\it stretching} mode at 645 cm$^{-1}$ presents an anomaly which
coincides with the magnetic transition temperature and signals a close
connection between magnetism and lattice in this material.
