The objective of this study was to investigate seasonal variation in the C, N and S stable isotope composition of retail organic and conventional Irish beef. A total of 242 beef samples (127 organic, 115 conventional) was collected in a one-year survey and analysed by isotope ratio mass spectrometry. The δ 13C time series in conventional beef was significantly non-random, with a pronounced seasonal positive shift of >2‰ between December and June, whilst δ 13C in organic beef was less variable and significantly more negative. In conventional beef, δ 15N was remarkably invariant (remaining close to 7‰) throughout the year, while organic beef was more variable and also significantly lower in δ 15N. The S isotope composition (δ 34S) exhibited a complex seasonal pattern in both types of beef. These results show that seasonal patterns can occur in the isotopic composition of beef, probably reflecting seasonality in animal feeding practices modulated by tissue turnover rates. Such seasonal variation needs to be considered in the isotopic authentication of beef and other animal-derived products.