This work presents the medium-resolution (R ~ 1500) spectroscopic follow-up of 522 low-metallicity star candidates from the Southern Photometric Local Universe Survey (S-PLUS). The objects were selected from narrowband photometry, taking advantage of the metallicity-sensitive S-PLUS colors. The follow-up observations were conducted with the Blanco and Gemini South telescopes, using the COSMOS and GMOS spectrographs, respectively. The stellar atmospheric parameters (T _eff, $\mathrm{log}\,g$ , and [Fe/H]), as well as carbon and α-element abundances, were calculated for the program stars in order to assess the efficacy of the color selection. Results show that ${92}_{-3}^{+2} \% $ of the observed stars have [Fe/H] ≤ -1.0, ${83}_{-3}^{+3} \% $ have [Fe/H] ≤ -2.0, and ${15}_{-3}^{+3} \% $ have [Fe/H] ≤ -3.0, including two ultra metal-poor stars ([Fe/H] ≤ -4.0). The 80th percentile for the metallicity cumulative distribution function of the observed sample is [Fe/H] = -2.04. The sample also includes 68 carbon-enhanced metal-poor stars. Based on the calculated metallicities, further S-PLUS color cuts are proposed, which can increase the fractions of stars with [Fe/H] ≤ -1.0 and ≤ -2.0 to 98% and 88%, respectively. Such high success rates enable targeted high-resolution spectroscopic follow-up efforts, as well as provide selection criteria for fiber-fed multiplex spectroscopic surveys.