<p>Gambling harm prevention messages are catching up with best practices from tobacco: Australia has implemented seven independently-designed messages, and the UK is due to make a similar change to replace industry-designed slogans. Three of the seven Australian messages are “self-appraisal” messages, such as “What’s gambling really costing you?”, that encourage people to reflect on their gambling behavior. These three Australian self-appraisal messages were tested against another three from the previous literature and four novel messages, using a mixed-methods approach on 3,993 UK-based gamblers. Message effectiveness was calculated in two ways: firstly, by the messages’ average rank across four relevant self-report outcomes (e.g., “this message makes me want to gamble less”) and secondly, by their proportion of participant comments that yielded positive sentiment. One novel message, “Would you be comfortable telling your family how much you lost gambling today?” ranked highest across both approaches. Nine messages ranked similarly across the two approaches, with only the Australian message “What are you really gambling with?” obtaining divergent rankings of tenth and fourth across the two approaches. Free-text responses highlighted a preference for short, punchy messages, with opinions divided regarding whether vague or concrete messages were better at prompting self-reflection. Messages all yielded more negative than positive sentiment, and only began to appear relevant to participants with higher Problem Gambling Severity Index scores. We inform a growing number of researchers and policymakers who are interested in implementing independently-designed gambling harm prevention messages, including self-appraisal messages.</p>