Key points Both endogenous opioids and opiate drugs of abuse modulate learning of habitual and goal‐directed actions, and can also modify long‐term plasticity of corticostriatal synapses. Striatal projection neurons of the direct pathway co‐release the opioid neuropeptide dynorphin which can inhibit dopamine release via κ‐opioid receptors. Theta‐burst stimulation of corticostriatal fibres produces long‐term potentiation (LTP) in striatal projection neurons when measured using whole‐cell patch recording. Optogenetic activation of direct pathway striatal projection neurons inhibits LTP while reducing dopamine release. Because the endogenous release of opioids is activity dependent, this modulation of synaptic plasticity represents a negative feedback mechanism that may limit runaway enhancement of striatal neuron activity in response to drugs of abuse. Abstract Synaptic plasticity in the striatum adjusts behaviour adaptively during skill learning, or maladaptively in the case of addiction. Just as dopamine plays a critical role in synaptic plasticity underlying normal skill learning and addiction, endogenous and exogenous opiates also modulate learning and addiction‐related striatal plasticity. Though the role of opioid receptors in long‐term depression in striatum has been characterized, their effect on long‐term potentiation (LTP) remains unknown. In particular, direct pathway (dopamine D1 receptor‐containing; D1R‐) spiny projection neurons (SPNs) co‐release the opioid neuropeptide dynorphin, which acts at presynaptic κ‐opioid receptors (KORs) on dopaminergic afferents and can negatively regulate dopamine release. Therefore, we evaluated the interaction of co‐released dynorphin and KOR on striatal LTP. We optogenetically facilitate the release of endogenous dynorphin from D1R‐SPNs in brain slice while using whole‐cell patch recording to measure changes in the synaptic response of SPNs following theta‐burst stimulation (TBS) of cortical afferents. Our results demonstrate that TBS evokes corticostriatal LTP, and that optogenetic activation of D1R‐SPNs during induction impairs LTP. Additional experiments demonstrate that optogenetic activation of D1R‐SPNs reduces stimulation‐evoked dopamine release and that bath application of a KOR antagonist provides full rescue of both LTP induction and dopamine release during optogenetic activation of D1R‐SPNs. These results suggest that an increase in the opioid neuropeptide dynorphin is responsible for reduced TBS LTP and illustrate a physiological phenomenon whereby heightened D1R‐SPN activity can regulate corticostriatal plasticity. Our findings have important implications for learning in addictive states marked by elevated direct pathway activation.