Can Muslim men wear sagging pants?

No, wearing pants that sag low enough to show your underwear or backside isn't allowed. A Muslim man has to cover from his navel to his knees, and sagging usually leaves part of that area exposed. A relaxed or baggy fit is fine, as long as the trousers sit at your waist.

The awrah problem

The majority of scholars say a man's awrah is from the navel to the knees. Sagging pants sit below the hips, so the area just under the navel and often the top of the backside is covered only by underwear, or not at all. Underwear that shows its shape or slips down doesn't count as proper covering. That makes the trend hard to square with Islamic dress, whatever its origins.

يَـٰبَنِىٓ ءَادَمَ قَدْ أَنزَلْنَا عَلَيْكُمْ لِبَاسًا يُوَٰرِى سَوْءَٰتِكُمْ وَرِيشًا ۖ وَلِبَاسُ ٱلتَّقْوَىٰ ذَٰلِكَ خَيْرٌ "O children of Adam, We have bestowed upon you clothing to conceal your private parts and as adornment. But the clothing of righteousness – that is best." Quran 7:26

Modesty is more than the minimum

The Prophet ﷺ said that modesty (haya) is part of faith (Sahih al-Bukhari 24). Even if someone wears long boxers or a long T-shirt to hide the gap, deliberately showing underwear is still a style built around exposure. Most scholars would call it against the spirit of modesty and good manners, especially in front of others.

Sagging and prayer

When you bow and prostrate, sagging pants pull down further. If your lower back or backside shows, your prayer may not be valid.
Pull your trousers up and use a belt before salah. See Can I pray in jeans?
If you're praying at home, the same rule applies. See What do I have to cover when praying at home alone?

What about the length at the ankle?

Sagging jeans often bunch up below the ankles. The Prophet ﷺ warned, "Whatever of the lower garment is below the ankles is in the Fire" (Sahih al-Bukhari 5787). Scholars differ on how this applies: many Shafi'i and Hanbali scholars say it's about arrogance, so long trousers without pride are disliked rather than forbidden, while others say men should keep trousers above the ankle in all cases. Wearing trousers at the waist and at a sensible length avoids both issues.

Baggy is fine

Loose clothes are actually closer to Islamic modesty than tight ones. Wide-leg jeans, cargo pants and joggers are all fine if they're worn at the waist. Islam isn't against streetwear; it's against exposure. For sports kit, the same navel-to-knee rule applies. See Can I still wear shorts to the gym as a Muslim?

Key points

Sagging that shows underwear or skin exposes the awrah.
Modesty is part of faith, beyond the bare minimum.
Check your trousers before prayer; use a belt.
Baggy clothes worn at the waist are fine.

Tip: You can keep your style without the sag: go for relaxed-fit or wide-leg trousers in a size that sits at your waist. You get the same loose streetwear look and stay covered all day, including in sujood.