Is dark skin on the neck a sign of diabetes or insulin resistance?

What you are describing sounds very much like acanthosis nigricans, and in most adults it is linked to insulin resistance. That means the body is producing more insulin than usual to keep sugars normal, and this high insulin stimulates the skin to become thicker and darker, especially on the neck and armpits.

Does it mean you definitely have diabetes?

Not always. But it is a strong clue that you should get screened for prediabetes and type 2 diabetes. Many people notice this skin change before diabetes is diagnosed.

Will it fade if you lose weight and improve control?

In many people, yes. When insulin resistance improves, the neck darkening often reduces gradually. It does not vanish overnight, but it can improve a lot with sustained weight loss and regular activity.

What should you do practically?

• Get tests: HbA1c, fasting glucose (or as your doctor advises), lipids, blood pressure
• Work on weight: even 5 to 10% weight loss can improve insulin resistance and reduce future diabetes risk
• Avoid harsh scrubbing. It irritates the skin and can worsen it
• If you want faster cosmetic improvement, a doctor may add creams like keratolytics or retinoids, but creams work best only when the root cause is addressed

Are there other causes? Yes, but less common.

• Certain medicines (like high-dose niacin or steroids) can contribute
• Hormonal conditions like PCOS can be associated
• Rarely, sudden, rapidly spreading acanthosis with weight loss or mouth involvement can suggest a serious internal illness, and needs urgent medical review

So your plan is simple. Treat this as a warning sign, not as a disaster. Screen once, then focus on lifestyle. The skin usually follows the metabolic improvement.