The recently synthesized acetylide compound KSeCCH containing the main group element selenium within the novel and in crystalline form unprecedented [SeCCH](-) anion was successfully investigated in the gas phase by high-resolution velocity-map imaging (VMI) and magnetic-bottle (MB) photoelectron spectroscopy coupled with an electrospray ionization source. Both VMI and MB spectra exhibited identical electron affinities (EA, 2.517 +/- 0.002 eV), spin-orbit coupling (SOC) splittings (1492 +/- 20 cm(-1)), and Se-C stretching frequencies (573 +/- 20 cm(-1)) of the corresponding neutral tetra-atomic radical [SeCCH](center dot) with the VMI spectrum possessing six times higher spectral resolution compared with the MB spectrum. These experimental values were well reproduced by calculations at the CCSD(T) level, in which both the isolated [SeCCH](-) anion and the [SeCCH](center dot) radical adopted linear geometries. The simulated spectra based on the calculated Franck-Condon factors, the SOC splitting, and the experimental line width matched well with the measured spectra. Furthermore, comparisons of the EA and SOC splitting values with the previously reported isolobal species [SeCN](center dot) are also made and discussed. The decrease in the EA and SOC splitting of [SeCCH](center dot) is ascribed to the differences in the electronegativities between C and N atoms as well as the electron density on the Se atom in its singly occupied molecular orbital (SOMO).