Portfolio management comprises of identifying, prioritizing, authorizing, managing, and controlling projects, programs, and other related work to achieve specific strategic business objectives. Utilizing a knowledge-based portfolio management approach can be a critical success factor for construction companies. This research aims to present a taxonomy to facilitate the learning process within a knowledge-based project portfolio management system. The taxonomy is capable of codification and classification of lessons revealed during life cycle of projects to enhance their retrieval. Within this context, following a detailed literature review process, the taxonomy is structured under four main categories as "project", "process", "actor", and "resource”. Categories provided in the taxonomy enable tagging of the lessons learned according to the intended level of detail, facilitate retrieval and reuse of the lessons learned in forthcoming projects. In this paper, we will present the structure of the proposed taxonomy and discuss how it can be used to improve portfolio management in construction.