Hyperlactatemia predicts mortality in patients with sepsis and septic shock, and its normalization is a potential treatment goal. We investigated the association of blood lactate and its changes over time with 90-day mortality in septic shock. We performed a post hoc analysis of 513 septic shock patients with admission blood lactate measurements in the prospective, observational, multicenter FINNAKI study. Repetitive lactate measurements were available in 496 patients for analyses of change in lactate values during intensive care unit stay. The 90-day mortality for all patients was 33.3%. Patients with admission lactate >2 mmol/L had higher 90-day mortality than those with admission lactate ≤2 mmol/L (43.4% vs. 22.6%, P < 0.001). Patients with persistent hyperlactatemia (>2 mmol/L) at ≥72 h had higher 90-day mortality compared with those with a lactate value of ≤2.0 mmol/L (52.0% vs. 24.3%, P < 0.001). Time-weighted mean lactate values were higher in non-survivors than in survivors, (median [IQR] 2.05 [1.38-4.22] mmol/L vs. 1.29 [0.98-1.77] mmol/L, P < 0.001). Time to normalization of lactate was comparable for 90-day non-survivors and survivors (median [IQR] 17.0 [3.5-43.5] vs. 15.0 [5.0-35.0] h, P = 0.67). In separate models, time-weighted mean lactate, lactate value at ≥72 h, and hyperlactatemia at ≥72 h were independently associated with 90-day mortality, but admission lactate and time to normalization of lactate were not. These findings may inform future clinical trials using combined surrogate endpoints for mortality in septic shock patients.