When searching for online casinos that accept Capital One Mastercard, it is important to consider the security measures in place. Look for sites that use encryption to protect your personal and financial information. Additionally, check for any fees or restrictions associated with using a Capital One Mastercard at online casinos. Some sites may have deposit or withdrawal limits when using this payment method. Overall, ensure that the online casino is reputable and trustworthy before making any transactions with your Capital One Mastercard.