A 42-year-old female patient was referred to our hospital with hypertension and hypokalemia and was diagnosed with primary aldosteronism. Dynamic contrast-enhanced computed tomography images revealed a 13-mm nodule on the lateral segment of the left adrenal gland and a fine venous connection between the nodule and the prominent renal capsular vein running nearby. The venograms in the left lateral tributary with a microcatheter confirmed alternative drainage to the left renal capsular vein during adrenal venous sampling, and the left renal capsular vein sampling was added. The patient was diagnosed with a left aldosterone-producing adenoma (APA) using the lateralization index (48.3) and a higher plasma aldosterone concentration (PAC) of the left lateral tributary (66 700 pg/mL) than other tributary samples after adrenocorticotropic hormone stimulation. Furthermore, markedly higher PAC (224 000 pg/mL) was observed in the left renal capsular vein blood than in the left adrenal central vein (45 000 pg/mL) and tributaries, confirming the diagnosis. Laparoscopic left partial adrenalectomy and following histopathological analysis revealed a CYP11B2-positive adrenocortical adenoma. Complete clinical and biochemical success for primary aldosteronism was achieved after 6 months. Direct evidence of APA blood venous drainage into the renal capsular vein has been demonstrated. Sampling from an alternative drainage pathway could be beneficial for APA diagnosis if such APA blood drainage is assumed. Aldosterone-producing adenomas may drain blood into an alternative pathway but for the adrenal vein. The presence of alternative venous drainage could be assumed by contrast-enhanced computed tomography or venogram during adrenal venous sampling. Sampling in the alternative drainage veins and demonstrating elevated aldosterone levels could help in diagnosing aldosterone-producing adenoma.