We develop a convenient, colorimetric assay (Au/PEI) for rapid iodide (I-) determination that can be prepared facilely by mixing commercially available chemicals including tetrachloroauric acid (HAuCl4) and poly(ether imide) (PEI), and the assay can be carried out directly by adding the samples to the assay without any pretreatment and additional procedure. Au/PEI operates on the principle that I- accelerates the formation of Au NPs, which leads to a visible color change from light yellow to red for naked-eye readout with high specificity. We integrate our assay on solid devices including gel hybrids (Au/PEI/GH) and filter paper (Au/PEI paper) to satisfy the demand of point-of-care testing and justify the practicality by detecting I- in lake water that was supplemented with 10, 20, or 40 μM of I-. Au/PEI/GH with the limit of detection of 0.35 μM can satisfy the detection of drinking water based on the guidelines (1.2 μM) set by the Chinese government, and Au/PEI paper can be used even after 1 year of storage. Such assays provide a convenient and straightforward choice for routine, on-site I- tests.