As was mentioned .net 3.5.1 DOES now support TLS 1.2; but you don't need the registry changes mentioned by @Paulina's answer.

I'm using VS 2008 with .net 3.5.30729.4926. All I had to do was:

Add imports:

Imports System.Security.Authentication
Imports System.Net

Add this to my code (C#):

public const SslProtocols _Tls12 = (SslProtocols)0x00000C00;
public const SecurityProtocolType Tls12 = (SecurityProtocolType)_Tls12;
ServicePointManager.SecurityProtocol = Tls12

VB.net version:

Const _Tls12 As SslProtocols = DirectCast(&HC00, SslProtocols)
Const Tls12 As SecurityProtocolType = DirectCast(_Tls12, SecurityProtocolType)
ServicePointManager.SecurityProtocol = Tls12

Culled from: https://support.microsoft.com/en-us/help/3154518/support-for-tls-system-default-versions-included-in-the-.net-framework Note: by defining the const in my code I could ignore everything else in the article including the registry edits and cs files.

As you can see from the docs, TLS 1.2 is not in the enumeration for SslProtocols, it was added to the enum in .NET 4.5 (thanks @orhun).

There is no workaround for TLS 1.2 compatibility on .NET 3.5.

Unfortunately you will have to upgrade to .NET 4.5 or later to get TLS 1.2 compatibility.

EDIT 10/11/17

My above answer is no longer accurate. In May of 2017, Microsoft released a package to allow TLS 1.2 in .NET 3.5.1.

You can make TLS 1.2 work with Framework 3.5.

Microsoft have released update for it.

Follow this steps

1. Install Support update for TLS in Framework 3.5 from here:

https://support.microsoft.com/en-us/help/3154518/support-for-tls-system-default-versions-included-in-the-.net-framework-3.5.1-on-windows-7-sp1-and-server-2008-r2-sp1

2. Go to registry
   • Type regedit in start
   • Right click and run as administrator
   • Navigate to registry keys

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727]

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v2.0.50727]

• Right click on the registry key and click Export

• Name the file and save it with .reg extension (keep them as your backup in case you need to restore them)

  3. Add entry to registry keys
• Make copy of the saved files and rename them
• Open with text editor and add following text to each key (this is for 64-bit operating system) and save changes (for 32-bit operating system look at the info in the link)

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727] "SystemDefaultTlsVersions"=dword:00000001

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v2.0.50727] "SystemDefaultTlsVersions"=dword:00000001

• Double click on the file and click Yes on the window to allow changes

  4. Add code to your project as specified in the link - Developer Guidance section

I applied this solution and it worked for me.