Background: We aimed to evaluate the effects of 0.3% carboxymethylcellulose (CMC) tear substitute treatment in dry eye disease (DED), as well as treatment compliance and adverse events (AEs). Methods: In this prospective, longitudinal study, a total of 30 eyes receiving 0.3% CMC tear substitute four times daily for DED were evaluated. Clinical endpoints included an ocular surface disease index (OSDI) questionnaire, average non-invasive tear film break-up time (A-NIBUT), lipid layer thickness (LLT), and a Schirmer test with anesthesia (ST). Treatment compliance and AEs were also assessed. All evaluations were performed at 2, 4, and 12 weeks of follow-up. Results: At the end of the follow-up, significant improvement was observed in all clinical endpoints with the following mean values: ΔOSDI questionnaire of −22.53 ± 14.68 points, ΔA-NIBUT of 4.81 ± 2.88 s, ΔLLT of 5.63 ± 6.53 nm, and ΔST of 2.8 ± 2.1 mm (p < 0.001 for all comparisons). Although repeated measures analysis showed that all clinical endpoints presented statistically significant differences (p < 0.001 for all comparisons LLTBaseline–LLT2-weeks (p = 0.460) and LLT4-weeks–LLT12-weeks (p = 0.071) were the only pairs of measures that reported non-statistically significant differences). In addition, treatment compliance was 94.3 ± 5.2% and transient AEs related to the use of 0.3% CMC tear substitute were reported. Conclusions: 0.3% CMC tear substitute treatment seems to achieve beneficial effects on the OSDI questionnaire, A-NIBUT, LLT, and ST. However, further studies at this concentration are needed to confirm these results.