Fires at the wildland–urban interface (WUI) result in the release of ash into the atmosphere that can be transported for long distances and deposited on land and in oceans. Wildfire ash has the potential to increase phytoplankton biomass in the open ocean by providing both major nutrients and trace metals. However, fires that originate at the WUI contain potentially toxic concentrations of metals such as Ti, Cr, Cu, Pb, and Zn, especially in coastal oceans close to WUI fires, where ash deposition rates are high. Here, we investigated the impact of fire ash from different sources originating from vegetation, structures, and vehicles on growth of the diatom Thalassiosira weissflogii (T. weissflogii). The diatom was exposed to ash suspensions containing equimolar concentrations of 10 and 50 µM Fe. The concentration of potentially toxic metals (e.g., Ti, Cu, and Zn) in the exposure suspensions decreased following the order vehicle ash suspension > structural ash suspension > vegetation ash suspension. Growth rates (GR) of T. weissflogii were between 0.44 d⁻¹ and 0.52 d⁻¹ in the controls, and varied with ash types, following the order vegetation (GR = 0.40 d⁻¹ to 0.48 d⁻¹) > vehicle (GR = 0.06 d⁻¹ to 0.46 d⁻¹) > structure (GR = 0.02 d⁻¹ to 0.31 d⁻¹) ash. Two ash samples (A 131 and A136) completely inhibited the growth of T. weissflogii, possibly due to high Ti, Cu, and Zn concentrations in the form of (nano)particles. Overall, this study showed that structural and vehicle ash, with high concentrations of potentially toxic metals, significantly suppress the growth of T. weissflogii, whereas vegetation ash with high concentrations of Fe and Mn but low concentrations of potentially toxic metals had no significant beneficial or suppressive effect. High concentrations of the metals Ti, Cu, and Zn in the form of nano(particles) in structural and vehicle ash are possible sources of toxicity to diatom growth. This study provides valuable insights into the potential impacts of WUI fires on aquatic ecosystems and can inform management strategies aimed at reducing these impacts.