Objective: To investigate the association between longitudinal changes in proton magnetic spectroscopy (MRS) metabolites and amyloid pathology in non-demented individuals; to explore the relationship between MRS and cognitive decline. Methods: In this longitudinal multiple time point study (a subset of the Swedish BioFINDER) we included cognitively healthy participants, individuals with subjective cognitive decline and mild cognitive impairment (MCI). MRS was acquired serially in 294 participants (670 individual spectra) from the posterior cingulate/precuneus. Using mixed-effects models, we assessed the association between MRS and baseline amyloid β42 (Aβ), and between MRS and longitudinal MMSE, accounting for APOE, age and sex. Results: While baseline MRS metabolites were similar in Aβ positive (Aβ+) and negative (Aβ-) individuals, in the Aβ+ group, the estimated rate of change was +1.9%/year for myo-inositol/creatine (mI/Cr) and -2.0%/year for N-acetyl-aspartate/myo-inositol (NAA/mI). In the Aβ- group, mI/Cr and NAA/mI yearly change was -0.05% and +1.2%, however this was not significant across time points. MCI Aβ+ group showed the steepest MRS changes, with an estimated rate of +2.93%/year (p=0.07) for mI/Cr and -3.55%/year (p<0.01) for NAA/mI. Further, in the entire cohort, we found that Aβ+ individuals with low baseline NAA/mI had a significantly higher rate of cognitive decline than Aβ+ individuals with high baseline NAA/mI. Conclusions: We demonstrate that the longitudinal change in mI/Cr and NAA/mI is associated with underlying amyloid pathology. MRS may be a useful non-invasive marker of Aβ-related processes over time. Further, we show that in Aβ+ individuals, baseline NAA/mI may predict the rate of future cognitive decline.