Background : Diabetes mellitus is a chronic disease requiring continuing self-management and support from family members. Family knowledge is relevant to the ability to provide appropriate support at home, while WhatsApp groups offer an accessible channel for structured health education. Objective : To assess changes in family knowledge regarding care of family members with diabetes mellitus before and after a one-week education program delivered through a WhatsApp group in the working area of Abeli Community Health Center, Kendari City. Methods​ : A quantitative pre-experimental one-group pretest-posttest study was conducted from May 5 to May 31, 2026. Sixty-nine family members were selected using purposive sampling. Knowledge was measured using a 23-item Diabetes Knowledge Questionnaire adapted to the family-care context. Educational materials were delivered once daily for seven consecutive days. The Wilcoxon signed-rank test was used for paired pre-post scores. Results : The mean knowledge score increased from 14.45 (SD 4.378) to 18.78 (SD 3.761), with a mean paired increase of 4.33 (SD 4.129). The median increased from 15.00 (IQR 6) to 20.00 (IQR 6). Sixty-one participants (88.4%) had higher posttest scores, three (4.3%) were unchanged, and five (7.2%) had lower scores. The Wilcoxon test showed a significant pre-post difference (Z = -6.150, p < 0.001), with r = 0.74. Conclusion : A statistically significant short-term improvement in family knowledge was observed after the seven-day WhatsApp-group education. Because there was no control group, the finding represents a within-group pre-post change rather than definitive evidence of a causal or sustained intervention effect.