Hey there, I’ve run into this exact snag before when working with Flask-SQLAlchemy in virtual environments—let’s work through some fixes step by step:

• Double-check your virtual environment activation
  It’s easy to assume the venv is active when it’s not. Look for your virtual environment’s name (like (venv)) at the start of your terminal prompt. If it’s missing, reactivate it:

  • Mac/Linux: source venv/bin/activate
  • Windows: venv\Scripts\activate
    To confirm it’s using the right pip, run which pip (Mac/Linux) or where pip (Windows)—the path should point inside your venv folder, not your global Python installation.

• Verify Flask-SQLAlchemy is installed in the active venv
  Even if you installed it earlier, packages can sometimes end up in the global environment by accident. Run pip list and look for Flask-SQLAlchemy in the output. If it’s missing, install it again with the venv active:

  pip install flask-sqlalchemy
  

  Pro tip: The installation uses a hyphen (flask-sqlalchemy), but the import uses an underscore (flask_sqlalchemy)—don’t mix those up!

• Switch to a script-based Flask startup
  Sometimes flask run uses the global Flask installation instead of your venv’s. Try creating a run.py file in your project root:

  from app import app  # Replace with your actual app import path
  
  if __name__ == "__main__":
      app.run(debug=True)
  

  Then launch it with python run.py—this ensures you’re using the Python interpreter from your active virtual environment.

• Validate your import in models.py
  Make sure your import line is spelled correctly:

  from flask_sqlalchemy import SQLAlchemy
  

  A common typo is using a hyphen instead of an underscore here (flask-sqlalchemy), which will immediately throw the module not found error.

• Last resort: Rebuild your virtual environment
  If none of the above works, your venv might be corrupted. Delete the existing venv folder and create a fresh one:

  # Mac/Linux
  rm -rf venv
  python -m venv venv
  source venv/bin/activate
  pip install flask flask-sqlalchemy sqlalchemy
  
  # Windows
  rmdir /s venv
  python -m venv venv
  venv\Scripts\activate
  pip install flask flask-sqlalchemy sqlalchemy
  

  Then try running your app again.

内容的提问来源于stack exchange，提问作者Pradeep