Dissolved organic nitrogen (DON) represents a significant reservoir of potentially plant available nitrogen (N) in saltmarsh ecosystems and wastewater treatment streams. Here we investigated the capacity of two halophytic plants used in wastewater remediation, Salicornia europaea and Aster tripolium, to access soluble organic forms of N when grown in water or soil-based culture. Incubation experiments with 15N13C- and 14C-labelled alanine and trialanine, 15NH4+ and 15NO3− indicated that both S. europaea and A. tripolium can take up and assimilate amino acids (alanine) and oligopeptides (trialanine), in addition to NH4+ and NO3−. In hydroponic solutions the uptake of alanine-N and trialanine-N was comparable to that of NO3−, but lower than NH4+. Comparison of the 13C and 15N ratios in the plant tissues suggested that at least 70% of the alanine and trialanine was taken up intact from solution. Plant capture of exogenously applied N by the soil-grown plants was much lower than in the hydroponically-grown plants. We ascribe this reduced rate of plant N uptake to increased microbial competition for DON, lower rates of N delivery to the root surface (from reduced mass flow and diffusion) and sorption of N solutes to the solid matrix. 14C-labelled amino acid and peptides indicated rapid assimilation of the DON once taken up by the plants. Our results strongly suggest that hydroponically grown S. europaea and A. tripolium are capable of taking up DON at high rates showing that it is highly pertinent to consider DON removal in the phytoremediation of wastewater. In addition, this is the first indication of direct peptide uptake by halophytic plants. We conclude that halophytes have evolved the capacity to take up a wide range of N compounds to satisfy their N demand when growing in N-limiting saltmarsh environments and that this intrinsic potential can be harnessed for wastewater treatment.