Background: Thrombocytopenia (TP), a common occurrence among patients admitted to the intensive care unit (ICU), is significantly associated with prolonged ventilator use, prolonged ICU stay, and increased mortality. The duration of TP serves as an indicator of patient outcome, although the exact duration of TP associated with poor patient outcome remains unclear. In this study, the data of 3,291 patients on their first admission to the ICU between January 2010 and December 2020 were retrospectively analyzed. Participants were divided into the no TP, TP 1-2 days, TP 3-6 days, and TP ≥7 days groups based on the duration of TP. External validation was performed using the Medical Information Mart for Intensive Care III data set. Results: A longer duration of TP was significantly associated with high volume of transfusion and high hospital mortality ( P < 0.01), and 37.3% of the participants developed TP during their ICU stay. The results of Kaplan-Meier survival analysis and Cox regression analysis after excluding the effects of patients who died shortly after ICU admission revealed the absence of significant differences between the no TP and TP 1-2 days groups ( P > 0.05). However, when the duration of TP exceeded 2 days, patient mortality increased with an increase in the duration of TP ( P < 0.01). Similar findings were obtained with the Medical Information Mart for Intensive Care III data set. Conclusions: The duration of TP in critically ill patients is positively correlated with poor patient outcome. We classified TP as either transient TP or persistent TP based on a cutoff duration of 2 days. Monitoring the duration of TP may aid in the prediction of patients' outcome in the ICU.