Background/Objectives: optimization of the therapeutic efficacy of antidiabetic agents is still a great challenge in the management of type 2 diabetes mellitus (T2DM), and attention is becoming more and more focused on the use of adjunctive agents that help target the drug action beyond the classic glycemic target. This study examined the effect of co-administration of an Artemisia herba-alba Asso (Asteraceae) aqueous extract on the glucose-lowering effect of dapagliflozin, a sodium–glucose cotransporter-2 (SGLT2) inhibitor. Methods: qualitative phytochemical screening and total phenolic and total flavonoid content were used to characterize the aqueous preparation. Male Wistar albino rats were divided into eight groups (10 rats each): four healthy and four diabetic rats induced with streptozotocin (STZ). Vehicle control, oral administration of an aqueous A. herba-alba preparation (0.39 g/kg twice daily), dapagliflozin (0.143 mg/kg/day), or a combination of both was given for 30 days. At baseline and days 7, 14, 21, and 30, glycated hemoglobin (HbA1c) was measured by nephelometry. Parallel fasting blood glucose (FBG) was monitored. Results: the diabetic animals treated with the combination regimen showed the greatest and most sustained decrease in HbA1c (from 4.42 ± 0.40% at baseline to 3.44 ± 0.17% at day 30; p < 0.001) and the greatest decrease in FBG (from 186.0 to 122.2 mg/dL). A. herba-alba monotherapy had little effect on HbA1c and had minimal effect on FBG (214.9 to 217.6 mg/dL); dapagliflozin monotherapy had a modest effect on HbA1c. The untreated diabetic controls (n = 10) showed a progressive increase in HbA1c from 4.45 ± 0.60% at 72 h post-induction to 11.90 ± 0.99% by day 30 (p < 0.001), consistent with delayed hemoglobin glycation relative to the rise in blood glucose. The repeated-measures ANOVA showed a highly significant interaction between time and treatment (p < 0.001). Conclusions: this study showed that the phytochemically defined aqueous A. herba-alba preparation is a beneficial adjunct to SGLT2 inhibitor therapy, an effect not observed with either treatment alone.