So I just updated my IntelliJ to 2025 version and this tabs for project stopped working. I tried this :

PS: This is for MacOS:

The Core Setting (As mentioned in answers above):

The primary setting that controls how IntelliJ IDEA opens new projects is still located here:

• Go to File > Settings (or IntelliJ IDEA > Preferences on macOS).

• Navigate to Appearance & Behavior > System Settings.

• In the "Project" section, look for the "Open project in" option.

  You typically have three choices:

  • New window: Always opens a new project in a separate window.

  • Current window: Closes the current project and opens the new one in the same window (this is not what you want for tabs).

  • Ask: Prompts you each time whether to open in a new window or the current one.

Crucially, none of these options directly say "Open as a new tab in the current window." This is where the confusion often arises.

IntelliJ IDEA's "projects as tabs" functionality is primarily achieved through macOS's native tabbed window support, rather than a direct IntelliJ setting for Windows/Linux.

1. IntelliJ IDEA Setting: Set "Open project in" to "New window" (or "Ask" and then choose "New window").

   • This is counter-intuitive, but for macOS to group them as tabs, IntelliJ needs to intend to open them in separate windows. macOS then intercepts this and merges them into tabs if your system settings allow.

2. macOS System Settings (The Key for Tabs):

   • Go to System Settings (or System Preferences on older macOS).

   • Navigate to Desktop & Dock (or Dock & Menu Bar on older macOS).

   • Scroll down to the "Windows" section.

   • Find "Prefer tabs when opening documents" and set it to "Always".