White rice is classified as having a low to high glycemic index (GI) depending on its amylose:amylopectin ratio. High GI food can be detrimental to diabetics since they can rapidly increase the glucose level in blood during digestion. Instant rice is increasing in popularity and is produced by dehydrating whole grain cooked rice, therefore, to make it more suitable for diabetics, the effect of the processes involved in the production on GI were tested. The objective was to optimize cooking temperature and the ratio of water to rice to reduce in vitro starch digestibility and GI of instant rice made from three Thai rice cultivars (Hom Mali (KDML105), Pathumthani Fragrant (PT) and Sao Hai (SH)) that had different amylose contents. Response surface methodology (RSM) was used to optimize the cooking conditions. The rapidly digestible starch (RDS) content and estimated glycemic index (eGI) decreased as cooking temperature decreased and the ratio of water to rice increased. The decrease in RDS content was associated with the increase in both slowly digestible starch (SDS) and resistant starch (RS) contents. The microstructure of instant rice kernels after cooking at higher temperature or lower volumes of water showed larger voids and a less compact surface, which facilitated enzymatic accessibility and increased starch digestibility. Rice cooked at higher temperature (90 °C) showed lower peak viscosity and breakdown in pasting properties, compared with lower temperature 82 °C. The optimal cooking condition for producing lower GI instant rice was cooking at 82 °C with 1.9‐fold water volume. Practical Application Rice has a high glycemic index (GI) that makes it unsuitable for consumption by many people with type 2 diabetes and obesity. Therefore, preparation of an instant rice product with a lower GI would ensure the product was beneficial to a wider range of the population.