Salt marsh islands are currently threatened by rates of erosion and sea level rise outpacing rates of sediment accretion. Past deposition of dredge materials was used to create spoil islands, which adds to total upland area but may result in different plant assemblages as found on natural islands. Plant diversity was compared between 11 spoil and 9 natural islands of the Timucuan Ecological and Historic Preserve, along a tributary of the St. Johns River, northeastern Florida. At each island, a 1 m² quadrat was used to estimate plant and unvegetated cover every 5 m along a transect that extended from the upland border to the approximate middle of each island. Three parallel transects were surveyed on each island. A total of 92 species was recorded with a mean of 21.8 species per spoil island and 16.7 species per natural island. Mean species richness was 3.0 ± 0.1 m⁻² and each species was present in < 3% of the sampled quadrats. Island area contributed to species richness in the preserve, describing 78.1% of the variability in species richness. Species diversity on spoil and natural islands was similar (ANOSIM; Clarke's r  =  0.069), indicating that the approximately 50-yr old spoil islands had sufficient time for physical conditions to be comparable to natural islands. The low species occurrences among islands were likely a response to local dynamics influencing species recruitment and survivorship. This study suggests that spoil material may serve to maintain upland habitat as rising sea level threatens coastal marshes.