How can I add a border to a widget in Flutter?

You can add the Text as a child to a Container that has a BoxDecoration with border property:

Container(
  margin: const EdgeInsets.all(15.0),
  padding: const EdgeInsets.all(3.0),
  decoration: BoxDecoration(
    border: Border.all(color: Colors.blueAccent)
  ),
  child: Text('My Awesome Border'),
)

The best way is using BoxDecoration()

Advantage

• You can set the border of a widget
• You can set the border Color or Width
• You can set a Rounded corner of a border
• You can add a Shadow of a widget

Disadvantage

• BoxDecoration only use with Container widget, so you want to wrap your widget in Container()

Example

    Container(
      margin: EdgeInsets.all(10),
      padding: EdgeInsets.all(10),
      alignment: Alignment.center,
      decoration: BoxDecoration(
        color: Colors.orange,
        border: Border.all(
            color: Colors.pink[800], // Set border color
            width: 3.0),   // Set border width
        borderRadius: BorderRadius.all(
            Radius.circular(10.0)), // Set rounded corner radius
        boxShadow: [BoxShadow(blurRadius: 10,color: Colors.black,offset: Offset(1,3))] // Make rounded corner of border
      ),
      child: Text("My demo styling"),
    )