To demonstrate the safety and feasibility of 2-fraction stereotactic body radiation therapy (SBRT) for prostate cancer.This single-center, noncomparative, phase 2/R-IDEAL 2b trial randomized 46 patients with intermediate/lower high-risk prostate cancer with visible gross tumor volume on multiparametric magnetic resonance imaging to receive 36.25 Gy in 5 fractions over 10 days or 24 Gy in 2 fractions with a gross tumor volume boost up to 27 Gy over 8 days. All treatment was delivered on a magnetic resonance linac with daily adaptive replanning. The primary endpoint was acute grade ≥2 (G2+) genitourinary (GU) toxicity (Common Terminology Criteria for Adverse Events version 5.0). Secondary endpoints include gastrointestinal (GI) toxicity and patient-reported outcomes.G2+ GU acute toxicity was observed in 6 of 22 patients (27.3%; 95% CI, 0.11-0.50) in the 2-fraction group and 7 of 24 patients (29.2%; 95% CI, 0.13-0.50) in the 5-fraction group. There were no grade 3 GU toxicities. G2+ urinary frequency rose from 4.5% (1 of 22) at week 2 to 13.6% (3 of 22) at week 4 in 2-fraction SBRT. G2+ urinary frequency peaked earlier in 5-fraction SBRT at 16.7% (4 of 24) in week 2, falling to 12.5% (3 of 24) at week 4. At 12 weeks, median Expanded Prostate Cancer Index Composite-26 urinary incontinence score was 85.5 (IQR, 75-100) for 2-fraction SBRT and 100 (IQR, 93.8-100) for 5-fraction SBRT. Urinary irritative-obstructive scores were higher at 12 weeks in the 2-fraction group (93.8; IQR, 87.5-100) than in the 5-fraction group (87.5; IQR, 81.3-93.8). Peak International Prostate Symptoms Score was lower in the 2-fraction group (8; IQR, 4-11) than in the 5-fraction group (13.5; IQR, 10-17). G2+ GI acute toxicity occurred in 3 of 24 (6.8%) after 5-fraction SBRT, but none after 2-fraction SBRT.Acceptable acute GU toxicity was seen after 2-fraction SBRT. Acute GI toxicity was low. Randomized trials are warranted to explore late toxicity and biochemical control.