diff --git a/base/intfuncs.jl b/base/intfuncs.jl
index f1e404ec77359..a85f37724b800 100644
--- a/base/intfuncs.jl
+++ b/base/intfuncs.jl
@@ -309,3 +309,10 @@ function digits{T<:Integer}(n::Integer, base::T=10, pad::Int=1)
 end
 
 isqrt(x::Integer) = oftype(x, trunc(sqrt(x)))
+
+function isqrt(x::Union(Int64,Uint64,Int128,Uint128))
+    s = oftype(x, trunc(sqrt(x)))
+    # fix with a Newton iteration, since conversion to float discards
+    # too many bits.
+    (s + div(x,s)) >> 1
+end
diff --git a/doc/stdlib/base.rst b/doc/stdlib/base.rst
index 4d111b6e9d81a..8894ecf0afa09 100644
--- a/doc/stdlib/base.rst
+++ b/doc/stdlib/base.rst
@@ -2551,9 +2551,9 @@ Mathematical Functions
 
    Return :math:`\sqrt{x}`. Throws ``DomainError`` for negative ``Real`` arguments. Use complex negative arguments instead.
 
-.. function:: isqrt(x)
+.. function:: isqrt(n)
 
-   Integer square root.
+   Integer square root: the largest integer ``m`` such that ``m*m <= n``.
 
 .. function:: cbrt(x)
 
diff --git a/test/math.jl b/test/math.jl
index 22ae859e166d3..2903713f03edf 100644
--- a/test/math.jl
+++ b/test/math.jl
@@ -173,3 +173,7 @@ end
 # Ensure subnormal flags functions don't segfault
 @test any(ccall("jl_zero_subnormals", Uint8, (Uint8,), 1) .== [0x00 0x01])
 @test any(ccall("jl_zero_subnormals", Uint8, (Uint8,), 0) .== [0x00 0x01])
+
+# isqrt (issue #4884)
+@test isqrt(9223372030926249000) == 3037000498
+@test isqrt(typemax(Int128)) == int128("13043817825332782212")