The implementation of 1,3 β-d-glucan (BDG) has been proposed as a diagnostic tool in antifungal stewardship programs (ASPs). We aimed to analyze the influence of serum BDG in an ASP for oncologic patients and solid organ transplant (SOT) recipients. We conducted a pre–post study. In the initial period (PRE), the ASP was based on bedside advice, and this was complemented with BDG in the post-period (POST). Performance parameters of the BDG assay were determined. Antifungal (AF) use adequacy was evaluated using a point score. Clinical outcomes and AF costs were also compared before and after the intervention. Overall, 85 patients were included in the PRE-period and 112 in the POST-period. Probable or proven fungal infections were similar in both groups (54.1% vs. 57.1%; p = 0.67). The determination of BDG contributed to improved management in 75 of 112 patients (66.9%). The AF adequacy score improved in the POST-period (mean 7.75 vs. 9.29; p < 0.001). Median days of empiric AF treatment was reduced in the POST-period (9 vs. 5 days, p = 0.04). All-cause mortality (44.7% vs. 34.8%; p = 0.16) was similar in both periods. The cost of AF treatments was reduced in the POST-period with a difference of 779.6 €/patient. Our data suggest that the use of BDG was a cost-effective strategy that contributed to safely improving the results of an ASP for SOT and oncologic patients.