Low temperature (LT) is one of the most prevalent factors limiting the productivity and geographical distribution of rice (Oryza sativa L.). Although significant progress has been made in elucidating the effect of LT on seed germination and reproductive development in rice, the genetic component affecting vegetative growth under LT remains poorly understood. Here, we report that rice cultivars harboring the dominant LTG1 (Low Temperature Growth 1) allele are more tolerant to LT (15–25°C, a temperature range prevalent in high‐altitude, temperate zones and high‐latitude areas), than those with the ltg1 allele. Using a map‐based cloning strategy, we show that LTG1 encodes a casein kinase I. A functional nucleotide polymorphism was identified in the coding region of LTG1, causing a single amino acid substitution (I357K) that is associated with the growth rate, heading date and yield of rice plants grown at LT. We present evidence that LTG1 affects rice growth at LT via an auxin‐dependent process(es). Furthermore, phylogenetic analysis of this locus suggests that the ltg1 haplotype arose before the domestication of rice in tropical climates. Together, our data demonstrate that LTG1 plays an important role in the adaptive growth and fitness of rice cultivars under conditions of low ambient temperature.