Objective: This study aimed to evaluate the effect of subcutaneous sterile water injection at the lumbosacral region on labour back pain. Methods: A quasi-experimental, pre/post test design was utilized to carry out this trial at the Labour and Delivery room, Mansoura University Hospital, Egypt. Sixty three primiparous in spontaneous active labour, indicated a low back pain ≥ 7 on numeric pain rating scale, and expected to have spontaneous vaginal delivery were included in this study as a one study group. Participants had received 4 subcutaneous sterile water injections of 0.1 ml at the lumbosacral region for once. Using three tools data were collected; the 1st was a structured interviewing questionnaire schedule to assess the participants basic characteristics, the 2nd was the numeric pain rating scale to evaluate the baseline pain intensity changes at ten minutes, one, two and three hours post injection, while the 3rd tool was the 5-points Likert scale for mother's satisfaction with the pain relief. Results: The baseline pain score was 8 ± 0.8. It was reduced by 2.5, 3.5, 4.5, and 5 points at 10 minutes, one, two, and three hours post injection respectively. Strong satisfaction with the used method for pain relief was reported by 87.3% of the mothers, while only 3.2% were dissatisfied. Conclusions: It can be concluded that subcutaneous sterile water injection is an effective labour back pain relief method. This leading the investigators to recommend; raising the awareness of the labour and delivery nurses about this method in order to implement it in practice.