In recent years a number of studies have used objective gambling data from online gambling operators to study gambling behavior. A few of these studies have compared gamblers’ actual gambling behavior (using account-based tracking data) with their subjective gambling behavior (using responses from survey data). The present study extended previous studies by comparing self-reported money deposited with the actual amount of money deposited. The authors were given access to an anonymized secondary dataset of 1,516 online gamblers from a European online gambling operator. After removing those who had not deposited any money in the previous 30 days, the final sample size for analysis was 639 online gamblers. The results indicated that gamblers were able to estimate fairly accurately how much money they had deposited in the past 30 days. However, the higher the amount of money deposited, the more likely gamblers underestimated the actual amount of money deposited. With respect to age and gender, there were no significant differences between male and female gamblers in their estimation biases. However, a significant age difference was found between those who overestimated and underestimated their deposits, with younger gamblers tending to overestimate their deposits. Providing feedback as to whether the gamblers overestimated or underestimated their deposits did not lead to any additional significant changes in the amount of money deposited when considering the overall reduction in deposits after self-assessment. The implications of the findings are discussed.