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The cyanobacterium Trichodesmiumis responsible for approximately half of the ocean's nitrogen input through nitrogen fixation. Although it was first recorded near Australia in the 18th century, the knowledge of where and when large quantity of Trichodesmiumaround Australia could be found is still lacking. Here, using multi‐band satellite imagery acquired between 2012 and 2021, we fill this knowledge gap through the use of deep learning, designed to recognize both the spectral shapes of individual pixels and spatial morphology of surface aggregations (scums) of Trichodesmium. Trichodesmiumscums were found nearly everywhere around Australia, with a cumulative footprint (i.e., where the 10‐year average density is >0.001‰) exceeding 4.6 million km2. Strong seasonality was found, with peak months between September and November. Furthermore, temperature, iron‐rich dust and black carbon aerosols, with the latter being a result of frequent bushfires, play major roles in determining the spatial distributions and seasonality of Trichodesmium. Responsible for half of the ocean's nitrogen input through nitrogen fixation, the saltwater cyanobacterium Trichodesmiumis ubiquitous in global tropical and subtropical oceans but particularly abundant around Australia. However, although the earliest report goes back to the 18th century, the knowledge of where and when large quantities of Trichodesmiumcan be found around Australia is still incomplete. Based on satellite imagery and deep learning, we quantified relative abundance of Trichodesmiumaround Australia for the period of 2012–2021. Surface aggregations of Trichodesmiumwere found almost everywhere except the southern coast, with a cumulative footprint exceeding 4.6 million km2. Strong seasonality was found, with peak months between September and November. The spatial distributions and seasonality were found to correlate well with water temperature, iron‐rich dust from Australian desert, and black carbon aerosols from frequent bushfires. With the projected ocean warming in the coming century, Trichodesmiummay expand further south, making the cumulative footprint even larger. Deep learning was applied to multi‐band satellite images to detect and quantify Trichodesmiumsurface scums around AustraliaTrichodesmiumscums were found nearly everywhere around Australia with a seasonality and a cumulative footprint exceeding 4.6 million km2Distribution and seasonality of Trichodesmiumwere driven by temperature, iron‐rich dust and black carbon from the mainland bushfires Deep learning was applied to multi‐band satellite images to detect and quantify Trichodesmiumsurface scums around Australia Trichodesmiumscums were found nearly everywhere around Australia with a seasonality and a cumulative footprint exceeding 4.6 million km2 Distribution and seasonality of Trichodesmiumwere driven by temperature, iron‐rich dust and black carbon from the mainland bushfires