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Boron (B) toxicity is increasingly being recognized as a problem in arid areas of West Asia, where lentil is widely grown. This study investigated whether or not: (1) phenotypic variation of B-toxicity tolerance exists at the seedling stage in lentil, (2) such variation persists to affect adult performance, and (3) there is geographical diversity in seedling tolerance. Four experiments were conducted in a greenhouse. Boric acid was added and mixed uniformly into the soil-mix to prepare high-B soil. Experiment I screened 77 lentil lines randomly selected from ICARDA’s international nurseries. Visual growth scores and foliar B-toxicity symptom scores were taken 4-5 weeks after sowing, and used as an indicator for B-toxicity tolerance. In Experiments II and III, four selected lines were studied beyond the seedling stage. In Experiment IV, 231 lentil accessions from seven different countries/regions of the world were screened. There were highly significant differences between the 77 lines in B-toxicity tolerance at the seedling stage as indicated by their degree of B-toxicity symptom, and plant growth. The two tolerant and two sensitive selections from Experiment I showed diverse performance up to maturity as reflected by their differences in leaflet loss, dry mass reduction, and seed yield when grown in high-B soil. There were significant differences in B-toxicity tolerance between the 231 accessions. On average, accessions from Afghanistan were the most tolerant, followed by those from India, Iraq, Syria, Europe, Ethiopia, and Nepal.