Mutations are the source of both genetic diversity and mutational load. However, the effects of increasing environmental temperature on plant mutation rates and relative impact on specific mutational classes (e.g., insertion/deletion indel vs. single nucleotide variant SNV) are unknown. This topic is important because of the poorly defined effects of anthropogenic global temperature rise on biological systems. Here, we show the impact of temperature increase on mutation, studying whole genome profiles of mutation accumulation (MA) lineages grown for 11 successive generations at 29°C. Whereas growth of at standard temperature (ST; 23°C) is associated with a mutation rate of 7 × 10 base substitutions per site per generation, growth at stressful high temperature (HT; 29°C) is highly mutagenic, increasing the mutation rate to 12 × 10 SNV frequency is approximately two- to threefold higher at HT than at ST, and HT-growth causes an ∼19- to 23-fold increase in indel frequency, resulting in a disproportionate increase in indels (vs. SNVs). Most HT-induced indels are 1-2 bp in size and particularly affect homopolymeric or dinucleotide A or T stretch regions of the genome. HT-induced indels occur disproportionately in nucleosome-free regions, suggesting that much HT-induced mutational damage occurs during cell-cycle phases when genomic DNA is packaged into nucleosomes. We conclude that stressful experimental temperature increases accelerate plant mutation rates and particularly accelerate the rate of indel mutation. Increasing environmental temperatures are thus likely to have significant mutagenic consequences for plants growing in the wild and may, in particular, add detrimentally to mutational load.