A series of new Pt(II) diimine complexes with different carbazolyl-capped acetylide ligands (Pt-1–Pt-5) were synthesized and characterized. Their photophysical properties were investigated systematically via spectroscopic and theoretical methods. All complexes exhibit ligand-centered ¹π,π* transitions in the UV region, and broad, structureless metal-to-ligand charge transfer (¹MLCT)/ligand-to-ligand charge transfer (¹LLCT) absorption bands in the visible spectral region. All complexes are emissive in solution at room temperature, with the emitting state being tentatively assigned to the ³MLCT/³LLCT states for Pt-1–Pt-4, and the emitting state of Pt-5 exhibiting a switch from the ³π,π* state in high-polarity solvents to the ³MLCT state in low-polarity solvents. Complexes Pt-1–Pt-5 all exhibit moderate triplet transient absorption (TA) from the visible to the NIR region, where reverse saturable absorption (RSA) could occur. The spectroscopic studies and theoretical calculations indicate that the photophysical properties of these Pt complexes can be tuned drastically by the carbazolyl-capped acetylide ligand, which would be useful for rational design of transition-metal complexes with high emission quantum yield, long excited-state lifetime, broadband excited-state absorption, and strong nonlinear transmittance for organic light-emitting and/or broadband nonlinear transmission applications.