Negative future thinking pervades emotional disorders. This hybrid efficacy-effectiveness trial tested a four-session, scalable online cognitive bias modification program for training more positive episodic prediction. 958 adults (73.3% female, 86.5% White, 83.4% from United States) were randomized to positive conditions with ambiguous future scenarios that ended positively, 50/50 conditions that ended positively or negatively, or a control condition with neutral scenarios. As hypothesized (preregistration: https://osf.io/jrst6), positive training participants improved in negative and positive expectancy bias, self-efficacy, and optimism more than control participants, ds and 97.5% CIs = -0.57 [-0.87, -0.27], 0.79 [0.42, 1.15], 0.28 [0.02, 0.53], 0.28 [0.04, 0.51], and, for expectancy bias, more than 50/50 participants, with gains maintained at 1-month follow-up. Unexpectedly, participants across conditions improved comparably in anxiety and depression symptoms and growth mindset. Targeting a transdiagnostic process with a scalable program may improve bias and outlook; however, further validation of outcome measures is required.