Betaine (tri-methyl glycine) is a natural compound found in sugar beets and serves as a methyl donor when fed to animals. Under heat stress it can act as an osmolyte, reducing dehydration. The objective was to evaluate the effect of feeding betaine-containing molasses on performance of transition dairy cows during late summer. In early September, cows were randomly assigned to betaine (B) or control (C) groups either at dry off (B: n = 10; C: n = 10) or 24 d before calving (B: n = 8; C: n = 8) based on their previous mature equivalent milk yield. Cows were fed a common diet supplemented either with a 28% CP molasses-based liquid supplement made from sugar cane or a 28% CP liquid supplement made of molasses from sugar cane (67%) and condensed beet solubles containing ~30% betaine (33%) for C and B cows, respectively, until 60 d in milk. The liquid supplement was fed at a rate of 1.1 and 1.4 kg DM/d for pre and postpartum cows, respectively. Before calving, all cows were housed in the same free-stall barn without supplemental cooling. After calving, all cows were cooled by misters and fans and were milked thrice daily. Feed intake was recorded daily and BW and BCS were assessed every 2 wk. Milk yield was recorded at each milking and composition was analyzed weekly. Blood samples were collected weekly from a subset of cows to assess hematocrit and concentrations of cortisol, betaine, and metabolites. There were no treatment effects on intake of DM and BW in the prepartum and postpartum periods, but cows fed B had lower (P = 0.05) BCS at 4 and 6 wk postpartum compared with C. For those cows enrolled at dry off, B tended to have lower (P < 0.15) hematocrit at 8 and 5 wk before calving compared with C, but no difference was detected postpartum. All cows fed B tended (P < 0.10) to produce more milk (44.2 vs. 41.5 kg/d) and 3.5% FCM (50.0 vs. 47.0 kg/d) compared with C, but no differences in milk composition were observed. In conclusion, supplementing transition cows with betaine-containing liquid supplement during late summer may prevent dehydration of dry cows based on a trend for lower hematocrit and increase 3.5% FCM yield in early lactation.