Numerical magnitude knowledge is related to current math abilities and predictive of future math learning. However, this relationship has previously been shown only for knowledge of positive magnitudes; the present study is the first, to our knowledge, to examine the connection between magnitude knowledge and math ability with negative integers. In the current study, 8th Grade Algebra I students (n = 154) were given assessments of their algebra and negative sign knowledge and 2 number line tasks, an all negative (-10,000 to 0) and a bidirectional scale (-1,000 to 1,000) as part of a pretest and posttest surrounding their unit on 2-step equation solving. Results show that students' negative sign and numerical magnitude knowledge are associated with their current, or pretest, algebra performance. However, only negative magnitude knowledge from the bidirectional number line was predictive of improvement in students' overall algebra knowledge. These findings illustrate differences between knowledge of positive and negative magnitudes and provide support for the integrated theory of numerical development.