Delta oscillations contribute to the human P300 event‐related potential evoked by oddball targets, although it is unclear whether they index contextual novelty (event oddballness, novelty P3, nP3), or target‐related processes (event targetness, target P3b). To examine this question, the electroencephalogram (EEG) was recorded during a cued task‐switching version of the Wisconsin card‐sorting test. Each target card was announced by a tone cueing either to switch or repeat the task. Novel sound distracters were interspersed among trials. Time‐frequency EEG analyses revealed bursts of delta (2–4 Hz) power associated with enhanced nP3 amplitudes to both task‐switch cues and novel distracters—but no association with target P3b. These findings indicate that the P300‐delta response indexes contextual novelty regardless of whether novelty emanates from endogenous (new task rules) or exogenous (novel distracters) sources of information.