AbstractElectrochemiluminescence (ECL) based on conjugated polymers or oligomers is persistently being pursued owing to its huge application scope ranging from ultra‐sensitive bioanalysis to ultra‐resolution imaging and spectroscopy. Because of the theoretical limit in radiative exciton generation yield (typically ∼25 %) of those polymers or oligomers, the corresponding ECL efficiency is still limited, which hampers its ECL performance and its related applications. Herein, we report ECL based on a thermally activated delayed fluorescence (TADF) polymer scaffold, which is characteristic of all‐exciton harvesting in the ECL process, and thus potentially capable of achieving ∼100 % ECL efficiency. These desired properties of the TADF polymer ECL is attributed to a fast and efficient up‐conversion process from non‐radiative triplet to radiative singlet states under thermal activation, which is absent in conventional fluorescent polymers/oligomers, such as F8BT. In this study, various ECL modes, including annihilation or co‐reactant mode using TPrA or S2O82− as co‐reactant, are confirmed for our model TADF polymer ECL system, which was different from fluorescent polymer ECL counterpart. Furthermore, solid‐state ECL sensing on L‐cysteine (an important marker of disease) is also evaluated by using the model TADF polymer. Ultralow detection limit in combination with high sensitivity and good specificity are achieved for this model system, indicative of a high potential of the TADF polymer scaffold for applications in the broad field of ECL.