Various low-volume bowel cleansing formulations that improve compliance have been approved and are being used in clinical practice. This study aimed to compare the effectiveness of 1 L polyethylene glycol (PEG) with ascorbic acid with that of sodium picosulfate (PICO) with magnesium citrate. This was a multicenter, randomized controlled, non-inferiority study. Patients were randomized into a 1 L PEG with ascorbic acid group and a PICO with magnesium citrate group according to the bowel cleansing agent used. Colonoscopy was performed as a single-blind study wherein the endoscopist had no information about any bowel preparation agent. The efficacy of bowel cleansing was assessed using the Harefield Cleansing Scale (HCS), and adverse events, preferences, and satisfaction were evaluated using a patient-reported questionnaire before colonoscopy. A total of 254 participants were randomly assigned to two groups: 115 in the 1 L PEG with ascorbic acid group and 113 in the PICO with magnesium citrate group. Overall bowel cleansing success was not statistically different between the two groups (97.4 vs. 97.3%), confirming that 1 L PEG with ascorbic acid was not inferior to PICO with magnesium citrate (lower confidence limit, -4.15%; p = 1.00). High-quality bowel cleansing was achieved in 87% of the 1 L PEG with ascorbic acid group and 77% of the PICO with magnesium citrate group (Lower confidence limit, 1.29%, p = 0.05). In terms of patient satisfaction, PICO with magnesium citrate was better, but compliance and side effects were similar in both groups. The 1 L PEG with ascorbic acid showed similar efficacy and adverse events as PICO with magnesium citrate. Although 1 L PEG with ascorbic acid is very effective in bowel preparation despite its small volume, it is necessary to increase satisfaction such as taste and feeling.