The authors present and validate the Patient Attachment Coding System (PACS), a transcript‐based instrument that assesses clients' in‐session attachment based on any session of psychotherapy, in multiple treatment modalities. One‐hundred and sixty clients in different types of psychotherapy (cognitive–behavioural, cognitive–behavioural‐enhanced, psychodynamic, relational, supportive) and from three different countries were administered the Adult Attachment Interview (AAI) prior to treatment, and one session for each client was rated with the PACS by independent coders. Results indicate strong inter‐rater reliability, and high convergent validity of the PACS scales and classifications with the AAI. These results present the PACS as a practical alternative to the AAI in psychotherapy research and suggest that clinicians using the PACS can assess clients' attachment status on an ongoing basis by monitoring clients' verbal activity. These results also provide information regarding the ways in which differences in attachment status play out in therapy sessions and further the study of attachment in psychotherapy from a pre‐treatment client factor to a process variable. Copyright © 2015 John Wiley & Sons, Ltd. Key Practitioner Message The Patient Attachment Coding System is a valid measure of attachment that can classify clients' attachment based on any single psychotherapy transcript, in many therapeutic modalities Client differences in attachment manifest in part independently of the therapist's contributions Client adult attachment patterns are likely to affect psychotherapeutic processes