Grocery retailers increasingly diversify their private label portfolios to cater to heterogeneous consumer needs and strengthen their retail brand equity. However, little is known about how consumers perceive the complementarity among private labels within a portfolio and how such perceptions affect their evaluations of the portfolio and the retail brand. This study examines the role of perceived complementarity in shaping consumer attitudes and loyalty toward private label portfolios and its subsequent spillover effects on retail brand image dimensions. Survey data from 257 participants in a Western European market revealed that perceived complementarity among private labels positively influences portfolio attitudes and loyalty. Furthermore, portfolio loyalty drives positive spillover effects on the retailer’s price, quality, and innovation image, highlighting the broader brand implications of private label strategies. These findings underscore the importance of designing complementary private label portfolios to enhance consumer evaluations and strengthen retail brand perceptions. Grocery retailers should strategically harness complementarity in their branding and communication activities.