Storage on smart devices such as smartphones and the Internet of Things has limited performance, capacity, and endurance. Deduplication has the potential to address these limitations by eliminating redundant I/Os and data, but it must be considered under the various resource constraints of the devices. This paper presents SmartDedup, a deduplication solution optimized for resource-constrained devices. It proposes a novel architecture that supports symbiotic in-line and out-of-line deduplication to take advantage of their complementary strengths and allow them to be adapted according to a device’s current resource availability. It also cohesively combines in-memory and on-disk fingerprint stores to minimize the memory overhead while achieving a good level of deduplication. SmartDedup is prototyped on EXT4 and F2FS and evaluated using benchmarks, workloads generated from real-world device images, and traces collected from real-world devices. The results show that SmartDedup substantially improves I/O performance (e.g., increases write and read throughput by 31.1% and 32%, respectively for an FIO experiment with 25% duplication ratio), reduces flash writes (e.g., by 70.9% in a trace replay experiment with 75.8% duplication ratio), and saves space usage (e.g., by 45% in a DEDISbench experiment with 46.1% duplication ratio) with low memory, storage, and battery overhead, compared to both native file systems and related deduplication solutions.