Can I achieve a more defined jawline through weight loss alone, or is surgery the only option? (Photo)

I suspect that you intend reducing the area under your chin rather than the jaw itself. Losing weight would definitely help however you may also benefit from neck liposuction, neck lift and possibly a chin implant to help restore contour. An in-person consultation and follow-up would be a good idea

With weight loss, you will see improvement in submental contour (below the chin), but I believe it will be limited.  To get better, more masculine jaw line, I believe that you will need to have neck lift, chin implant, and possibly jaw implant to give more definition to the angle of jaw.  I recommend you to see a board certified plastic surgeon for further discussion.

Hi there, 
Thank you for the inquiry. Losing weight reduces fat all over the body including the face. However, you might not be able to achieve the jawline definition you are looking for with weight loss alone. Lower face enhancement can be achieved with a combination of treatments including a neck liposuction procedure to help define the jawline, and/or with a chin implant which can create a more pronounced jawline with the increased projection of the chin. Based on your images, you may benefit from both procedures.
If you are unsure of the method that you would like to proceed with, look for an experienced, Board-certified facial plastic surgeon who implements advanced photo software system in their clinic, so you can see a 3D image of what you would potentially look like after the above mentioned procedures. 
Good luck!