<p>The present preregistered study investigated cognitive processes during deception about planned behavior. It extends prior research on deception about attitudes by addressing individual life plans as a more specific and immediate determinant of future behavior. By examining deception about individually planned behaviors, the present study establishes a link to previous forensic research focusing on deception concerning (criminal) behavior. Participants (<i>N</i> = 114) indicated whether they planned certain behaviors. Stimuli were selected individually for each participant. Participants responded more slowly and less accurately for deceptive than truthful responses. Medial frontal negativities (MFNs) were enlarged for deceptive compared to truthful responses, revealing greater response conflicts during lying. Decreased pre-response positivities (PRP) and late positive components (LPCs) for deceptive responses suggested that lies were accompanied by strategic monitoring and were cognitively more demanding, respectively. For all ERP components, the difference between deceptive and truthful responses was greater for planned than for unplanned behavior, revealing that lying about planned behavior was cognitively more challenging. Practice, i.e., repeating the task, did not affect the difference in ERPs between deceptive and truthful responses, indicating that deception did not become less cognitively demanding. The patterns of MFN and LPCs were stable across individual differences in Machiavellianism. The MFN, PRP, and LPC components appear to be promising markers of the additional executive processes involved in lying about individual future life plans. These findings provide a foundation for investigating ERP components during deception also regarding other nonforensic, everyday topics.</p>