The last two day-night games of the five-match series are scheduled to be played in Colombo on Saturday and Monday with rain forecast for both days.
"Sri Lanka Cricket is pleased to announce that a reserve day has been allocated for the fourth and fifth one-day internationals against Pakistan," the SLC said in a statement.
Rain has already affected the series, reducing the opening match in Pallekele to a 42-overs-a-side affair and restricting the third game in Colombo to just 6.2 overs before it was abandoned.
The series is currently tied 1-1, with Pakistan winning the opening match by six wickets and Sri Lanka taking the second by 76 runs.
The one-dayers will be followed by a three-Test series, which opens in Galle on June 22. AFP