Methods, apparatus, systems and articles of manufacture to perform identity matching across audience measurement systems are disclosed. An example apparatus includes a data normalizer to normalize audience measurement events corresponding to media exposure data obtained from a first audience measurement system and a second audience measurement system, a tree builder to build a k-dimensional tree based on normalized audience measurement events, a candidate identifier to calculate a distance between a query event and a first audience measurement event included in a search space in the k-dimensional tree, identify the query event and the first audience measurement event as a candidate match when the calculated distance satisfies a distance threshold, an array builder to generate metrics at an identifier-level based on a plurality of candidate matches, and a thresholder to identify an identifier mapping linking a first user identifier to a second user identifier when the metrics satisfy respective metric thresholds.