Background Although hypertension is becoming more prevalent among the adult population of the Lao People’s Democratic Republic (PDR), with a prevalence of approximately 20% in 2013, treatment adherence of patients with hypertension, especially those in rural areas, remains poorly understood. The objective of the present study was to examine the rate of medication adherence to antihypertensive medicines among outpatients with hypertension in rural districts of the Savannakhet. Methods A prospective observational study was conducted in Xepon, Phin, and Nong districts. The study population was outpatients aged 18 years or older who were prescribed antihypertensive medicines at three district hospitals between February and August 2017. Data were collected on the first day of treatment (day 0) and the day of follow-up (around day 7) through interviews with the patients and outpatient registration books. The medication adherence rate was determined using the four-item Morisky Medication Adherence Scale. The level of adherence was evaluated by the sum of the scale, with scores ranging from 0 to 4 points. The adherent group, namely those with a score of 0, and the non-adherent group, namely those with scores of 1 to 4 points, were compared. Fisher’s exact test was used to identify factors associated with medication adherence. Results Of the 68 patients examined, 38.2% newly began treatment. Half of the patients (n = 34, 50.0%, 95% CI 0.38 to 0.62) adhered to medication instructions. The adherent group was more likely to have received written instructions when prescribed medication, compared to the non-adherent group (79.4% vs 55.9%, p = 0.068). Those who perceived that hypertension needs lifelong treatment were significantly more likely to adhere to the medication regimen (p = 0.028). Conclusions Medication adherence was present among 50% of outpatients with hypertension who visited a district hospital. Therefore, providing written instructions to patients would be effective for improving medication adherence.