Antimony (Sb) is an emerging contaminant and until recently it was assumed to behave in a similar way to arsenic (As). Arsenic and Sb often co-occur in contaminated sites, yet most investigations consider their toxicity to plants singly. More research is needed to understand the interactions between As and Sb in soils and plants. This study investigated the interactive effect of As and Sb in terms of soil bioavailability, plant toxicity and bioaccumulation on the commercially important agricultural plant, water spinach (Ipomoea aquatica) using a pot experiment. Plants were exposed to As and Sb individually (As (individual), Sb (individual)) and as a mixture (As + Sb (combined)) at different concentrations. Plant growth was measured using shoot and root dry mass, length and chlorophyll a content of leaves. At the end of the bioassay, bioavailable metalloids were extracted from the soil as per a sequential extraction procedure (SEP) and plant tissue was analysed for metalloid content. For As, there were no differences observed between the bioavailability of As in the As + Sb (combined) and As (individual) treatments. For Sb, no increase in bioavailability was observed with co-contamination compared to single-Sb exposures for most concentrations except at 1250 mg/kg. Single-Sb was not toxic to I. aquatica shoot dry mass and length, but there was greater shoot Sb accumulation in the As + Sb (combined) than the Sb (individual) treatment. In contrast, single-As was toxic to I. aquatica growth. When As and Sb were present together in the soil, there was a synergistic toxicity to shoot dry mass (EC50 Toxic Unit (TU) was less than 1) and additive toxicity (EC50 equal to 1 TU) to shoot length. This work shows that the co-occurrence of As and Sb in soil increases Sb bioavailability and can cause synergistic toxicity to an important agricultural crop.