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A search for the K+→ π+ X decay, where X is a long-lived feebly interacting particle, is performed through an interpretation of the K+→ π+ νν analysis of data collected in 2017 by the NA62 experiment at CERN. Two ranges of X masses, 0–110 MeV/c 2 and 154–260 MeV/c 2, and lifetimes above 100 ps are considered. The limits set on the branching ratio, BR (K+→ π+ X), are competitive with previously reported searches in the first mass range, and improve on current limits in the second mass range by more than an order of magnitude.