As the late 1980's and early 1990's marked the period where the first countries started to see their population decline (all of them on Eastern Europe), I compiled the population evolution for the past 30 years. The year of reference might vary (1988-1991) from country to country. The list brings all countries that are already declining or about to:
LATVIA -------------- -29.0% ----- 1,893,223 ----- 2,666,567
BULGARIA ------------ -27.4% ----- 6,520,314 ----- 8,981,446
GEORGIA ------------- -26.2% ----- 4,019,400 ----- 5,443,359
LITHUANIA ----------- -23.5% ----- 2,810,761 ----- 3,674,802
BOSNIA-HERZEGOVINA -- -22.0% ----- 3,415,752 ----- 4,377,033
CROATIA ------------- -18.7% ----- 3,888,529 ----- 4,784,265
ROMANIA ------------- -15.9% ---- 19,186,201 ---- 22,810,035
ESTONIA ------------- -15.0% ----- 1,330,068 ----- 1,565,662
UKRAINE ------------- -14.9% ---- 43,999,924 ---- 51,706,742
ALBANIA ------------- -11.1% ----- 2,829,741 ----- 3,182,417
ARMENIA -------------- -9.9% ----- 2,963,300 ----- 3,287,677
SERBIA --------------- -9.3% ----- 6,871,547 ----- 7,576,837
PUERTO RICO ---------- -9.3% ----- 3,193,694 ----- 3,522,037
BELARUS -------------- -8.3% ----- 9,349,645 ---- 10,199,709
HUNGARY -------------- -5.8% ----- 9,772,756 ---- 10,374,823
RUSSIA --------------- -2.5% --- 143,759,445 --- 147,400,537
POLAND --------------- +0.4% ---- 38,265,013 ---- 38,119,000
JAPAN ---------------- +2.1% --- 126,146,099 --- 123,611,167
CZECH REPUBLIC ------- +2.2% ---- 10,524,167 ---- 10,302,215
SLOVAKIA ------------- +3.3% ----- 5,449,270 ----- 5,274,335
GERMANY -------------- +4.3% ---- 83,155,031 ---- 79,753,227
ITALY ---------------- +4.4% ---- 59,257,566 ---- 56,778,031
GREECE --------------- +4.8% ---- 10,718,565 ---- 10,223,392
PORTUGAL ------------- +4.9% ---- 10,347,892 ----- 9,867,147
CUBA ----------------- +4.9% ---- 11,181,595 ---- 10,662,000
TAIWAN -------------- +14.3% ---- 23,561,236 ---- 20,605,831
SOUTH KOREA --------- +19.4% ---- 51,829,136 ---- 43,410,899
SPAIN --------------- +22.1% ---- 47,450,795 ---- 38,872,268
THAILAND ------------ +24.6% ---- 67,993,000 ---- 54,548,530
CHINA --------------- +24.8% - 1,411,778,724 - 1,130,822,993
UNITED STATES ------- +33.3% --- 331,449,281 --- 248,709,873
And some random regions, provinces/states, cities/metro areas that are either from those countries or also experience decline:
DETROIT (city) ------------------- -37.8% ------- 639,111 ----- 1,028,067
ALBANIA (sans Tirana & Durres) --- -35.4% ----- 1,625,522 ----- 2,514,659
BULGARIA (sans Sofia) ------------ -35.2% ----- 5,039,484 ----- 7,782,831
LATVIA (sans Riga-Pieriga) ------- -34.8% ------- 899,623 ----- 1,379,153
SAXONY-ANHALT (state) ------------ -27.5% ----- 2,180,684 ----- 3,008,000
CLEVELAND (city) ----------------- -26.3% ------- 372,624 ------- 505,629
CENTRAL-NORTH UKRAINE (region) --- -25.4% ----- 5,500,076 ----- 7,372,555
CROATIA (sans Zagreb) ------------ -24.3% ----- 2,817,379 ----- 3,723,450
DONBAS (region) ------------------ -24.1% ----- 6,221,602 ----- 8,195,129
ST. LOUIS (city) ----------------- -24.0% ------- 301,578 ------- 396,685
RIGA-PIERIGA (region) ------------ -22.8% ------- 993,600 ----- 1,287,414
THURINGIA (state) ---------------- -22.0% ----- 2,120,237 ----- 2,718,000
AKITA (prefecture) --------------- -21.8% ------- 959,502 ----- 1,227,478
BALTIMORE (city) ----------------- -20.4% ------- 585,708 ------- 736,016
EAST GERMANY (sans Berlin) ------- -18.8% ---- 12,499,707 ---- 15,394,000
PITTSBURGH (city) ---------------- -18.1% ------- 302,971 ------- 369,962
GENOA (city) --------------------- -17.7% ------- 558,930 ------- 678,771
ROMANIA (sans Bucharest) --------- -17.6% ---- 16,863,240 ---- 20,455,525
SOUTHERN UKRAINE (sans Crimea) --- -16.8% ----- 9,301,758 ---- 11,176,206
BELARUS (sans Minsk) ------------- -16.0% ----- 7,093,099 ----- 8,439,599
NAGASAKI (prefecture) ------------ -16.0% ----- 1,312,317 ----- 1,562,959
BUFFALO (city) ------------------- -15.2% ------- 278,349 ------- 328,233
SERBIA (sans Beograd) ------------ -14.6% ----- 4,995,513 ----- 5,846,281
SAN JUAN (urban area) ------------ -13.2% ----- 1,005,081 ----- 1,157,961
GENOA (metro area) --------------- -12.1% ----- 1,085,682 ----- 1,235,496
TURIN (city) --------------------- -11.9% ------- 848,196 ------- 962,507
SHIKOKU (region) ----------------- -11.9% ----- 3,696,171 ----- 4,195,069
TOHOKU (region) ------------------ -11.6% ----- 8,611,195 ----- 9,738,285
LIGURIA (region) ------------------ -9.9% ----- 1,509,805 ----- 1,676,282
HUNGARY (sans Budapest) ----------- -8.9% ----- 6,881,213 ----- 7,550,031
RUSSIA (sans Moscow) -------------- -5.0% --- 131,512,239 --- 138,435,315
PITTSBURGH (metro area) ----------- -3.9% ----- 2,370,930 ----- 2,468,289
SICILIA (region) ------------------ -2.5% ----- 4,840,876 ----- 4,966,386
SOUTHERN ITALY (region) ----------- -2.4% ---- 20,052,901 ---- 20,537,484
GALICIA (region) ------------------ -1.3% ----- 2,695,645 ----- 2,731,669
BUCHAREST (metro area) ------------ -1.3% ----- 2,322,961 ----- 2,354,510
TURIN (metro area) ---------------- -1.1% ----- 2,212,996 ----- 2,236,765
BUFFALO (metro area) -------------- -1.9% ----- 1,166,902 ----- 1,189,288
WEST VIRGINIA (state) ------------- +0.0% ----- 1,793,716 ----- 1,793,477
OSAKA-KOBE-KYOTO (metro area) ----- +0.6% ---- 20,541,441 ---- 20,414,233
ZAGREB (metro area) --------------- +1.0% ----- 1,071,150 ----- 1,060,815
CLEVELAND (metro area) ------------ +1.1% ----- 2,790,470 ----- 2,759,823
NAPLES (metro area) --------------- +1.4% ----- 5,410,526 ----- 5,337,254
BUDAPEST (metro area) ------------- +2.4% ----- 2,891,543 ----- 2,824,792
SOUTH KOREA (sans Seoul region) --- +3.9% ---- 25,787,831 ---- 24,826,761
ATHENS (metro area) --------------- +4.1% ----- 3,742,235 ----- 3,594,817
DETROIT (metro area) -------------- +4.5% ----- 5,325,319 ----- 5,095,695
BERLIN-BRANDENBURG (metro area) --- +4.6% ----- 6,195,159 ----- 5,925,000
ST. PETERSBURG (city) ------------- +7.2% ----- 5,384,342 ----- 5,023,506
BEOGRAD (metro area) -------------- +8.4% ----- 1,876,034 ----- 1,730,556
ST. LOUIS (metro area) ----------- +10.5% ----- 2,754,124 ----- 2,492,525
MILAN (metro area) --------------- +12.0% ----- 8,136,590 ----- 7,266,404
ROME (metro area) ---------------- +12.4% ----- 4,227,588 ----- 3,761,067
BERLIN (city) -------------------- +12.4% ----- 3,664,088 ----- 3,260,000
TOKYO (metro area) --------------- +13.3% ---- 43,653,441 ---- 38,543,517
WEST GERMANY (sans Berlin) ------- +13.4% ---- 66,991,236 ---- 59,064,333
KIEV (city) ---------------------- +13.8% ----- 2,962,180 ----- 2,602,754
LISBON (metro area) -------------- +13.9% ----- 2,871,133 ----- 2,520,708
BALTIMORE (metro area) ----------- +19.0% ----- 2,794,636 ----- 2,348,221
TAIPEI (metro area) -------------- +22.5% ----- 9,269,756 ----- 7,566,329
BARCELONA (metro area) ----------- +23.4% ----- 5,743,402 ----- 4,654,407
SOFIA (metro area) --------------- +23.5% ----- 1,480,830 ----- 1,198,615
MINSK (metro area) --------------- +28.2% ----- 2,256,546 ----- 1,760,110
MOSCOW (city) -------------------- +36.6% ---- 12,249,227 ----- 8,967,232
MADRID (metro area) -------------- +37.0% ----- 6,779,888 ----- 4,947,555
SEOUL (metro area) --------------- +40.1% ---- 26,043,325 ---- 18,586,128