The effect of platform temperature combined with a T5 heat treatment on the fatigue life of additively manufactured aluminum alloy AlSi7Mg was characterized and understood. High-cycle fatigue tests were carried out on samples built with four platform temperatures (35 °C, 60 °C, 80 °C and 200 °C) and post-processing heat treatment strategies (F and T5). Microstructural and fractographic observations combined with microhardness measurements were performed. A log-normal statistical distribution regressed with 90% B-basis probabilities of survival revealed that specimens produced on a platform maintained at 80 °C and post-processed with a T5 heat treatment presented the highest fatigue life among the conditions tested. Precipitation of silicon within the aluminum cells during the T5 heat treatment is the proposed explanation for the improved fatigue life of the T5 samples. In the as-built condition, specimens produced at 200 °C were found to be less resistant to fatigue than the specimens built at lower temperatures. The coarser microstructure and lowest microhardness resulting from high-temperature manufacturing explain this reduced fatigue strength. All fatigue cracks initiated from manufacturing discontinuities. This led to a fatigue life prediction model based upon linear elastic fracture mechanics. The model was fitted to the experimental results of the F and T5 samples separately. With the exception of the 35 °C—T5 specimens, the predicted fatigue lives agree with the experimental results and literature.