Introduction and Hypothesis <p>Low back pain may play a contributing role in the emergence or worsening of stress urinary incontinence (SUI) in women, by affecting neuromuscular control and pelvic stability. This study aimed to examine the relationship between SUI, musculoskeletal performance, and physical activity level (PAL) in women with nonspecific low back pain (NSLBP) by comparing them with a healthy control group.</p> Methods <p>One hundred fifty-two women were assigned to either the healthy control (no NSLBP) group (<i>n</i> = 76) or the NSLBP group (<i>n</i> = 76). Pain severity was evaluated using the Visual Analog Scale (VAS), the severity of SUI was detected using the International Consultation on Incontinence Questionnaire Short-Form (ICIQ-SF), musculoskeletal performance was measured using the trunk flexor endurance test (TFET), wall sit-hold test (WSHT), 3-minute burpee test (3-MBT), and sit and reach test (SRT), respectively. International Physical Activity Questionnaire-Long Form (IPAQ-LF) assessed the PAL. The Mann–Whitney U test was used to compare groups, and Spearman correlation analysis determined the intra-group correlation levels.</p> Results <p>The NSLBP group had significantly higher ICIQ-SF, lower TFET, 3-MBT, and SRT scores (<i>p</i> = 0.016). In the healthy control group, ICIQ-SF was not associated with any musculoskeletal performance test parameters and IPAQ-LF (<i>p</i> &gt; 0.05). While a significant positive correlation was found between IPAQ-LF and VAS during activity (<i>r</i> = 0.266, <i>p</i> = 0.020), ICIQ-SF had a negative correlation with TFET (<i>r</i> = −0.308, <i>p</i> = 0.007), 3-MBT (<i>r</i> = −0.320, <i>p</i> = 0.005), and IPAQ-LF (<i>r</i> = −0.298, <i>p</i> = 0.009) in the NSLBP group.</p> Conclusions <p>As NSLBP gets worse, SUI complaints increase. Trunk flexor muscle endurance, agility, and physical activity levels decrease as the severity of SUI increases in women with NSLBP.</p>