We present a timing analysis of PSR J1602–5100 using approximately seven years of observations from the Parkes 64-m radio telescope. A slow glitch that occurred between 2008 September 24 and 2010 May 14 (MJDs 54733 and 55330) is identified. During this time, the pulsar showed a slow exponential growth in the spin frequency ν $\nu$, and the spin-down rate |ν˙| suddenly decreased to a peak value, followed by a linear return to its initial value. Our measurements of the maximum Δν and Δν˙ are 176nHz and 3.58×10−15s−2 $3.58\times 10^{-15}~\text{s}^{-2}$, corresponding to fractional sizes of 152×10−9 (Δν/ν) $(\Delta\nu/\nu)$ and −38.6×10−3 (Δν˙/ν˙) $(\Delta\dot{\nu}/\dot {\nu})$, respectively. This is the largest slow glitch observed so far. Moreover, more complex changes in the shape of the pulse profile are considered to be associated with this unusual glitch activity.