diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4972caf14..33da09d44 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@
 
 ### Improvements
  * `ERC777`: `_burn` is now internal, providing more flexibility and making it easier to create tokens that deflate. ([#1908](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1908))
+ * `ReentrancyGuard`: greatly improved gas efficiency by using the net gas metering mechanism introduced in the Istanbul hardfork. ([#1992](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1992))
 
 ### Breaking changes
  * `ERC165Checker` now requires a minimum Solidity compiler version of 0.5.10. ([#1829](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1829))
diff --git a/contracts/utils/ReentrancyGuard.sol b/contracts/utils/ReentrancyGuard.sol
index 4015cdec6..7804893af 100644
--- a/contracts/utils/ReentrancyGuard.sol
+++ b/contracts/utils/ReentrancyGuard.sol
@@ -11,6 +11,9 @@ pragma solidity ^0.5.0;
  * `nonReentrant` may not call one another. This can be worked around by making
  * those functions `private`, and then adding `external` `nonReentrant` entry
  * points to them.
+ *
+ * _Since v2.5.0:_ this module is now much more gas efficient, given net gas 
+ * metering changes introduced in the Istanbul hardfork.
  */
 contract ReentrancyGuard {
     // counter to allow mutex lock with only one SSTORE operation
@@ -34,5 +37,6 @@ contract ReentrancyGuard {
         uint256 localCounter = _guardCounter;
         _;
         require(localCounter == _guardCounter, "ReentrancyGuard: reentrant call");
+        _guardCounter = 1;
     }
 }