How can I URL encode a string in Excel VBA?

No, nothing built-in (until Excel 2013 - see this answer).

There are three versions of URLEncode() in this answer.

• A function with UTF-8 support. You should probably use this one (or the alternative implementation by Tom) for compatibility with modern requirements.
• For reference and educational purposes, two functions without UTF-8 support:
  • one found on a third party website, included as-is. (This was the first version of the answer)
  • one optimized version of that, written by me

A variant that supports UTF-8 encoding and is based on ADODB.Stream (include a reference to a recent version of the "Microsoft ActiveX Data Objects" library in your project):

Public Function URLEncode( _
   ByVal StringVal As String, _
   Optional SpaceAsPlus As Boolean = False _
) As String
  Dim bytes() As Byte, b As Byte, i As Integer, space As String

  If SpaceAsPlus Then space = "+" Else space = "%20"

  If Len(StringVal) > 0 Then
    With New ADODB.Stream
      .Mode = adModeReadWrite
      .Type = adTypeText
      .Charset = "UTF-8"
      .Open
      .WriteText StringVal
      .Position = 0
      .Type = adTypeBinary
      .Position = 3 ' skip BOM
      bytes = .Read
    End With

    ReDim result(UBound(bytes)) As String

    For i = UBound(bytes) To 0 Step -1
      b = bytes(i)
      Select Case b
        Case 97 To 122, 65 To 90, 48 To 57, 45, 46, 95, 126
          result(i) = Chr(b)
        Case 32
          result(i) = space
        Case 0 To 15
          result(i) = "%0" & Hex(b)
        Case Else
          result(i) = "%" & Hex(b)
      End Select
    Next i

    URLEncode = Join(result, "")
  End If
End Function

This function was found on freevbcode.com:

Public Function URLEncode( _
   StringToEncode As String, _
   Optional UsePlusRatherThanHexForSpace As Boolean = False _
) As String

  Dim TempAns As String
  Dim CurChr As Integer
  CurChr = 1

  Do Until CurChr - 1 = Len(StringToEncode)
    Select Case Asc(Mid(StringToEncode, CurChr, 1))
      Case 48 To 57, 65 To 90, 97 To 122
        TempAns = TempAns & Mid(StringToEncode, CurChr, 1)
      Case 32
        If UsePlusRatherThanHexForSpace = True Then
          TempAns = TempAns & "+"
        Else
          TempAns = TempAns & "%" & Hex(32)
        End If
      Case Else
        TempAns = TempAns & "%" & _
          Right("0" & Hex(Asc(Mid(StringToEncode, _
          CurChr, 1))), 2)
    End Select

    CurChr = CurChr + 1
  Loop

  URLEncode = TempAns
End Function

I've corrected a little bug that was in there.

I would use more efficient (~2× as fast) version of the above:

Public Function URLEncode( _
   StringVal As String, _
   Optional SpaceAsPlus As Boolean = False _
) As String

  Dim StringLen As Long: StringLen = Len(StringVal)

  If StringLen > 0 Then
    ReDim result(StringLen) As String
    Dim i As Long, CharCode As Integer
    Dim Char As String, Space As String

    If SpaceAsPlus Then Space = "+" Else Space = "%20"

    For i = 1 To StringLen
      Char = Mid$(StringVal, i, 1)
      CharCode = Asc(Char)
      Select Case CharCode
        Case 97 To 122, 65 To 90, 48 To 57, 45, 46, 95, 126
          result(i) = Char
        Case 32
          result(i) = Space
        Case 0 To 15
          result(i) = "%0" & Hex(CharCode)
        Case Else
          result(i) = "%" & Hex(CharCode)
      End Select
    Next i
    URLEncode = Join(result, "")
  End If
End Function

Note that neither of these two functions support UTF-8 encoding.

The accepted answer's code stopped on a Unicode error in Access 2013, so I wrote a function for myself with high readability that should follow RFC 3986 according to Davis Peixoto, and cause minimal trouble in various environments.

Note: The percent sign itself must be replaced first, or it will double-encode any previously encoded characters. Replacing space with + was added, not to conform with RFC 3986, but to provide links that don't break due to formatting. It is optional.

Public Function URLEncode(str As Variant) As String
    Dim i As Integer, sChar() As String, sPerc() As String
    sChar = Split("%|!|*|'|(|)|;|:|@|&|=|+|$|,|/|?|#|[|]| ", "|")
    sPerc = Split("%25 %21 %2A %27 %28 %29 %3B %3A %40 %26 %3D %2B %24 %2C %2F %3F %23 %5B %5D +", " ")
    URLEncode = Nz(str)
    For i = 0 To 19
        URLEncode = Replace(URLEncode, sChar(i), sPerc(i))
    Next i
End Function

The VBA-tools library has a function for that:

http://vba-tools.github.io/VBA-Web/docs/#/WebHelpers/UrlEncode

It seems to work similar to encodeURIComponent() in JavaScript.

The best of both worlds. This function uses the new(ish) worksheet function ENCODEURL() if the workbook is open in Excel 2013 or newer.

If it's an older version of Excel then this function uses htmlfile instead.

You can also force this function to use htmlfile by passing True as the optional bForceOldSchool argument.

Function URLEncode$(s$, Optional bForceOldSchool As Boolean)
  Select Case True
    Case bForceOldSchool Or Val(Application.Version) < 15
               URLEncode = CreateObject("htmlfile").parentWindow.EncodeUriComponent(s)
    Case Else: URLEncode = WorksheetFunction.EncodeURL(s)
  End Select
End Function