Selective attention refers to the ability to focus on goal-relevant information while fltering out irrelevant information. In a multisensory context, how do people selectively attend to multiple inputs when making categorical decisions? Here, we examined the role of selective attention in cross-modal categorization in two experiments. In a speed categorization task, participants were asked to attend to visual or auditory targets and categorize them while ignoring other irrelevant stimuli. A response-time extended multinomial processing tree (RT-MPT) model was implemented to estimate the contribution of attentional focusing on task-relevant information and attentional fltering of distractors. The results indicated that the role of selective attention was modality-specifc, with diferences found in attentional focusing and fltering between visual and auditory modalities. Visual information could be focused on or fltered out more efectively, whereas auditory information was more difcult to flter out, causing greater interference with task-relevant performance. The fndings suggest that selective attention plays a critical and diferential role across modalities, which provides a novel and promising approach to understanding multisensory processing and attentional focusing and fltering mechanisms of categorical decision-making.