Singlet fission (SF) is a spin‐allowed conversion process, in which one singlet exciton is replaced by a pair of triplet excitons resided on adjacent molecules. To rationalize this phenomenon, a multiexcitonic spin‐zero triplet‐pair state is generally hypothesized as an intermediate state for SF. However, the details of its formation during the fission process, and its subsequent evolution over time are still remain unclear. Thus, this work provides a model of the SF system for triisopropylsilylethynyl‐pentacene by using femtosecond time‐resolved transient absorption spectroscopy. A mixed state of singlet and multiexciton, which mediates the ultrafast SF process is directly observed. This work represents a new viewpoint for the research on SF intermediates.