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Submesoscale flows are energetic motions on scales of several kilometers that may lead to substantial vertical motions. Here we present satellite and ship radar as well as underway conductivity‐temperature‐depth and Acoustic Doppler Current Profiler observations of a cyclonic submesoscale filament in the marginal ice zone of Fram Strait. The filament created a 500‐m thin and 50‐km long sea ice streak and extends to >250‐m depth with a negative/positive density anomaly within/below the halocline. The frontal jets of 0.5 m/s are in turbulent thermal wind balance while the ageostrophic secondary circulation in places appears to subduct Atlantic Water at >50 m/day. Our study reveals the submesoscale dynamics related to sea ice shapes that can be sensed remotely and shows how submesoscale dynamics contribute to shaping the marginal ice zone. It also demonstrates the co‐occurrence and mixing of water masses over short horizontal scales, which has implications for ocean and sea ice models and understanding of patch formation of planktonic organisms. A sea ice streak in the marginal ice zone was observed with radar measurements. Below this streak in situ shipboard measurements of the temperature, salinity, and velocity field revealed a cyclonic submesoscale filament. This is a line of denser water of a few kilometers width bounded by strong counteracting velocities. This denser water is also associated with a different water mass and thus a change in biological properties and communities. This provides in situ confirmation for previous theoretical conclusions of how oceanic flows on kilometer scales structure the sea ice and biology in the marginal ice zone. The understanding of such small‐scale processes helps improve computer models of the ocean and sea ice dynamics. It also makes it possible to interpret oceanic flows from remote sensing of sea ice. Furthermore, it gives indication over which horizontal scales biological processes vary in the ocean. A submesoscale cyclonic filament in the marginal ice zone was observed with multiple techniquesThe ageostrophic circulation accumulated sea ice and had local subduction of more than 50 m/dayAlthough previously unresolved, such filaments impact mixing, sea ice, and biological productivity