Emerging online marketing strategies are an opportunity for the sport sponsorship industry as a way of complementing traditional methods. However, in-depth attention has not been given to the study of congruence effects on the effectiveness of sponsorship of sport event websites, and specifically to study the role and effects of sponsor logos. The main aim of this study was to evaluate the congruence effect of sponsor brands featured on the website of a sports event on sponsorship effectiveness in visual, attitude-related and behavioural terms, using an eye-tracker to monitor memory activation and changes in attitudes and intentions. In study 1, the role of congruence on website sponsorship was analysed, using real brands sponsoring the ninth edition of the “Mallorca 312” Cycletourist Tour (42 participants). In study 2, the congruence of fictitious brands was analysed on the effects of website sponsorship of the 37th edition of the MAPFRE (competitions brand name) Copa del Rey regatta (101 participants). Congruence is preferable to incongruence in sponsor brands, except when the sponsorship aims to boost a recall of new market brands. The results validate the importance of managing congruence levels in the online sponsorship of sports events due to the influence on sponsorship effectiveness and its impact on cognitive processing.