We compared the morphology of gully sedimentary fans on Svalbard as possible analogs to gullies on Mars in order to constrain whether fluvial and/or debris-flow processes are predominantly responsible for the formation of Martian gullies. Our analysis is based on high-resolution imagery (High Resolution Stereo Camera [HRSC-AX], similar to 20 cm/pixel) acquired through a flight campaign in summer 2008 and ground truth during two expeditions in the summers of 2008 and 2009 in Svalbard, compared to high-resolution satellite imagery (High Resolution Imaging Science Experiment [HiRISE], similar to 25 cm/pixel) from Mars. On Svalbard, fluvial and debris-flow processes are evident in the formation of gullies, but the morphological characteristics clearly show that the transport and sedimentation of eroded material are predominated by debris flows. Most investigated gullies on Mars lack clear evidence for debris-flow processes. The Martian gully fan morphology is more consistent with the deposition of small overlapping fans by multiple fluvial flow events. Clear evidence for debris flows on Mars was only found in one new location, in addition to a few previously published examples. The occurrence of debris-flow processes in the formation of Martian gullies seems to be rare and locally limited. If predominantly fluvial processes caused the formation of gullies on Mars, then large amounts of water would have been required for their formation because of the relatively low sediment supply in stream and/or hyperconcentrated flows. Repeated seasonal or episodic snow deposition and melting during periods of higher obliquity in the recent past on Mars can best explain the formation of the gullies.