diff --git a/NEWS.md b/NEWS.md
index fd68ecce6d5ed..ee9345f0e077b 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -581,6 +581,9 @@ Library improvements
     like other `AbstractDict` subtypes and its constructors mirror the
     ones of `Dict`. ([#25210])
 
+  * `IOBuffer` can take the `sizehint` keyword argument to suggest a capacity of
+    the buffer ([#25944]).
+
 Compiler/Runtime improvements
 -----------------------------
 
diff --git a/base/iobuffer.jl b/base/iobuffer.jl
index 422092f9ccd18..b19b9b3a782bd 100644
--- a/base/iobuffer.jl
+++ b/base/iobuffer.jl
@@ -40,6 +40,7 @@ It may take optional keyword arguments:
 - `read`, `write`, `append`: restricts operations to the buffer; see `open` for details.
 - `truncate`: truncates the buffer size to zero length.
 - `maxsize`: specifies a size beyond which the buffer may not be grown.
+- `sizehint`: suggests a capacity of the buffer (`data` must implement `sizehint!(data, size)`).
 
 When `data` is not given, the buffer will be both readable and writable by default.
 
@@ -84,10 +85,14 @@ function IOBuffer(
         write::Union{Bool,Nothing}=nothing,
         append::Union{Bool,Nothing}=nothing,
         truncate::Union{Bool,Nothing}=nothing,
-        maxsize::Integer=typemax(Int))
+        maxsize::Integer=typemax(Int),
+        sizehint::Union{Integer,Nothing}=nothing)
     if maxsize < 0
         throw(ArgumentError("negative maxsize: $(maxsize)"))
     end
+    if sizehint !== nothing
+        sizehint!(data, sizehint)
+    end
     flags = open_flags(read=read, write=write, append=append, truncate=truncate)
     buf = GenericIOBuffer(data, flags.read, flags.write, true, flags.append, Int(maxsize))
     if flags.truncate
@@ -101,8 +106,9 @@ function IOBuffer(;
         write::Union{Bool,Nothing}=true,
         append::Union{Bool,Nothing}=nothing,
         truncate::Union{Bool,Nothing}=true,
-        maxsize::Integer=typemax(Int))
-    size = maxsize == typemax(Int) ? 32 : Int(maxsize)
+        maxsize::Integer=typemax(Int),
+        sizehint::Union{Integer,Nothing}=nothing)
+    size = sizehint !== nothing ? Int(sizehint) : maxsize != typemax(Int) ? Int(maxsize) : 32
     flags = open_flags(read=read, write=write, append=append, truncate=truncate)
     buf = IOBuffer(
         StringVector(size),
diff --git a/base/strings/basic.jl b/base/strings/basic.jl
index a747ac39e5b8e..6e83d9f263f4b 100644
--- a/base/strings/basic.jl
+++ b/base/strings/basic.jl
@@ -512,8 +512,7 @@ isascii(s::AbstractString) = all(isascii, s)
 ## string map, filter, has ##
 
 function map(f, s::AbstractString)
-    out = IOBuffer(StringVector(sizeof(s)), read=true, write=true)
-    truncate(out, 0)
+    out = IOBuffer(sizehint=sizeof(s))
     for c in s
         c′ = f(c)
         isa(c′, Char) || throw(ArgumentError(
@@ -525,8 +524,7 @@ function map(f, s::AbstractString)
 end
 
 function filter(f, s::AbstractString)
-    out = IOBuffer(StringVector(sizeof(s)), read=true, write=true)
-    truncate(out, 0)
+    out = IOBuffer(sizehint=sizeof(s))
     for c in s
         f(c) && write(out, c)
     end
diff --git a/base/strings/io.jl b/base/strings/io.jl
index 97dbf4eed89e6..86009bd456e27 100644
--- a/base/strings/io.jl
+++ b/base/strings/io.jl
@@ -81,10 +81,7 @@ julia> sprint(showcompact, 66.66666)
 ```
 """
 function sprint(f::Function, args...; context=nothing, sizehint::Integer=0)
-    s = IOBuffer(StringVector(sizehint), read=true, write=true)
-    # specialized version of truncate(s,0)
-    s.size = 0
-    s.ptr = 1
+    s = IOBuffer(sizehint=sizehint)
     if context !== nothing
         f(IOContext(s, context), args...)
     else
@@ -99,11 +96,11 @@ tostr_sizehint(x::Float64) = 20
 tostr_sizehint(x::Float32) = 12
 
 function print_to_string(xs...; env=nothing)
+    if isempty(xs)
+        return ""
+    end
     # specialized for performance reasons
-    s = IOBuffer(StringVector(tostr_sizehint(xs[1])), read=true, write=true)
-    # specialized version of truncate(s,0)
-    s.size = 0
-    s.ptr = 1
+    s = IOBuffer(sizehint=tostr_sizehint(xs[1]))
     if env !== nothing
         env_io = IOContext(s, env)
         for x in xs
@@ -436,8 +433,7 @@ Returns:
 function unindent(str::AbstractString, indent::Int; tabwidth=8)
     indent == 0 && return str
     # Note: this loses the type of the original string
-    buf = IOBuffer(StringVector(sizeof(str)), read=true, write=true)
-    truncate(buf,0)
+    buf = IOBuffer(sizehint=sizeof(str))
     cutting = true
     col = 0     # current column (0 based)
     for ch in str
diff --git a/base/strings/util.jl b/base/strings/util.jl
index 9fe05fe6edcf5..9d6839665d053 100644
--- a/base/strings/util.jl
+++ b/base/strings/util.jl
@@ -383,9 +383,7 @@ function replace(str::String, pat_repl::Pair; count::Integer=typemax(Int))
     i = a = firstindex(str)
     r = coalesce(findnext(pattern,str,i), 0)
     j, k = first(r), last(r)
-    out = IOBuffer(StringVector(floor(Int, 1.2sizeof(str))), read=true, write=true)
-    out.size = 0
-    out.ptr = 1
+    out = IOBuffer(sizehint=floor(Int, 1.2sizeof(str)))
     while j != 0
         if i == a || i <= k
             unsafe_write(out, pointer(str, i), UInt(j-i))