Purpose: This study aims to clarify the concept, structure, and soteriological purpose of sati practice in Early Buddhism on the basis of Pāli sources, and to analyze the core transformations that occur when sati is transposed into contemporary clinical mindfulness programs. It pays particular attention to the function of sati within jhāna practice, an aspect largely neglected in existing scholarship. Methods: The study combines philological analysis of the Pāli Nikāyas and the Visuddhimagga with a review of the manuals and outcome literature of five representative mindfulness-based programs (MBSR, MBCT, DBT, ACT, MSC), and then conducts a conceptual comparison across five axes: teleology, technique, secularization, the concept of self, and the sati-samādhi linkage. Results: Canonical sati operates as a retentive and ethically discriminating faculty embedded in the Eightfold Path, and functions in jhāna through access support, object retention, purification, and linkage to insight. Contemporary mindfulness selectively extracts present-moment observation and non-reactivity while detaching sati from sīla, samādhi, and the Four Noble Truths; the asubha dimension of body contemplation and the sati–samādhi linkage are the two elements most completely lost. Conclusion: The relation between the two traditions is best described neither as simple continuity nor as mere distortion, but as a selective extraction that yields demonstrable clinical benefit at the cost of the transformative depth secured by the original practice framework. The study further proposes a working definition of sati grounded in the canonical sources, and argues—through an examination of the conditions under which the notion of upāya (skillful means) properly applies—that this selective extraction constitutes an independent reconstruction serving a different telos, rather than either upāya or distortion.