Non-native species have invaded coastal systems worldwide, altering community structures and ecosystem functioning. One of the most widely distributed marine invaders is the kelp Undaria pinnatifida. In Australasia, Undaria is a large annual kelp that typically has a unimodal growth pattern characterized by high cover during late-winter and spring. It is possible that Undaria co-occurs with mussels and impacts the biodiversity of mussel assemblages on rocky reefs, especially given its presence on mussel farms. Here, we tested whether Undaria and mussels co-occurred on rocky reefs at different temporal and spatial scales, and whether Undaria affects the diversity of mussel-associated small mobile invertebrates. Analyses of survey data showed that individuals of Undaria are often attached to, or interspersed around, mussel aggregations in the low intertidal zone where Undaria was found at similar abundances in its typical winter growth and summer senescence seasons. We hypothesize that this unexpected pattern is caused by overlapping generations coupled with longer growing seasons, potentially triggered by localized higher wave action, rather than individual plants persisting for an entire year. Analyses of Undaria holdfasts and non-living mimics of Undaria holdfasts, showed that live and large holdfasts, in particular, alter and increase small-scale diversity of mobile invertebrates where Undaria co-occurs with mussels. We conclude that Undaria has a longer temporal presence where it co-occurs with mussels and that Undaria alter mussel-associated communities of small mobile invertebrates.