Context: Reductions in objectively measured moderate to vigorous physical activity (MVPA) have been reported among individuals with anterior cruciate ligament reconstruction (ACLR). Self-reported measures of physical activity are commonly used to assess participation in physical activity after ACLR despite the lack of evidence to support the validity of such measures within this population. Objective: The objective of this research was to determine the relationships between objectively measured MVPA, self-reported physical activity, and knee function among individuals with ACLR. Setting: University laboratory. Patients (or Other Participants): Thirty-one participants with a history of ACLR (sex: 23 females and 8 males; age = 19.8 [1.4] y) and 31 matched controls (sex: 23 females and 8 males; age = 20.6 [1.7] y) enrolled in this study. Intervention(s): None. Main Outcome Measures: Participants completed self-reported physical activity using the Tegner Activity Scale and the Marx Activity Rating Scale. Participant MVPA was objectively measured using an ActiGraph wGT3X-BT accelerometer for a 7-day period during which the monitor was worn for not less than 10 hours per day. Primary outcome measures were the amount of time spent in MVPA (minutes per week) and time spent in MVPA performed in bouts of ≥10 minutes (minutes per week). Relationships between the Tegner Activity Score, Marx Activity Rating Scale, and objectively measured MVPA variables were assessed using partial Spearman’s rank correlation coefficients after controlling for activity monitor wear time. Results: There were no significant relationships between objectively measured MVPA and self-reported physical activity (ρ ≤ 0.31, P ≥ .05) or self-reported knee-related function (ρ ≤ .41, P ≥ .05) among ACLR participants. Conclusions: Objectively measured physical activity is not significantly related to self-reported physical activity or self-reported knee function among individuals with a history of ACLR. Consideration of objective and self-reported physical activity within this population may provide key insights into disconnects between perception and the reality of physical activity engagement following ACLR.